Posts Tagged ‘Integer Sequences’

Further Variants of the “Look-and-Say” Sequence

January 13th, 2011

In two previous posts, I explored Conway’s famous “look-and-say” sequence 1, 11, 21, 1211, 111221, 312211, …, obtained by repeatedly describing the sequence’s previous term, as well as a simple binary variant of the sequence. In this post I will use similar techniques to explore some further variations of the sequence – a version where each term in the sequence is read in ternary, and a related sequence where no digit larger than 2 may be used when describing its terms.

As with the regular look-and-say sequence, the way we will attack these sequences is by constructing a “periodic table” of elementary non-interacting subsequences that all terms in the sequence are made up of. Then standard recurrence relation techniques will allow us to determine the rate of growth of the length of the terms in the sequences as well as the limiting distribution of the different digits in the sequence.

The Ternary Look-and-Say Sequence

Since we have already looked at the regular (i.e., decimal) look-and-say sequence, which is equivalent to the base-4 version of the sequence since it never contains a digit of 4 or larger, and we have also looked at the binary version of the sequence, it makes sense to ask what happens in the intermediate case of the ternary (base-3) version of the sequence: 1, 11, 21, 1211, 111221, 1012211, … (see A001388).

As always, we begin by listing the noninteracting subsequences that make this version of the sequence tick. Not surprisingly, it is more complicated than the corresponding table (of 10 subsequences) in the binary case, but not as complicated as the corresponding table (of 92 subsequences) in the decimal case.

# Subsequence Evolves Into
1 1 (3)
2 10 (5)
3 11 (19)
4 110 (21)
5 1110 (2)(4)
6 111210 (2)(8)
7 111221 (2)(16)
8 1121110 (22)(4)
9 112211 (23)
10 112221 (21)(20)
11 11222110 (21)(24)
12 1122211210 (21)(25)
13 1211 (7)
14 121110 (6)(4)
15 1221 (9)
16 12211 (10)
17 122110 (11)
18 1221121110 (12)(4)
19 21 (13)
20 211 (15)
21 2110 (17)
22 211210 (18)
23 212221 (14)(20)
24 22110 (26)
25 221121110 (27)(4)
26 222110 (2)(24)
27 22211210 (2)(25)

The (27×27) transition matrix for this evolution rule is included in the text file at the end of this post. Its characteristic polynomial is

The maximal eigenvalue of the transition matrix is thus the largest root of x3 – x – 1, which is approximately 1.324718. It follows that the number of digits in the terms of this sequence grows on average by about 32.5% from one term to the next.

The Look-and-Say Sequence with Digits 1 and 2

Closely related to the ternary version of the sequence is the sequence obtained by reading the previous term in the sequence, but with the restriction that you can never use a number larger than 2 (see A110393). This sequence begins 1, 11, 21, 1211, 111221, 21112211, …, and the sixth term is obtained by reading the fifth term as “two ones, one one, two twos, one one”. Because only two different digits appear in this sequence, it is perhaps not surprising that its table of noninteracting subsequences is quite simple:

# Subsequence Evolves Into
1 1 (2)
2 11 (5)
3 111 (7)
4 1211 (3)(6)(1)
5 21 (4)
6 22 (6)
7 2111 (1)(6)(3)

The transition matrix associated with this evolution rule is

As before, the average rate of growth of the number of digits in the terms of this sequence is determined by the magnitude of the largest eigenvalue of this matrix. A simple calculation reveals that this eigenvalue is √φ = 1.272…, where φ = (1 + √5)/2 is the golden ratio. Furthermore, we can answer the question of how many 1s there are in the terms of this sequence compared to 2s by looking at the eigenvector corresponding to the maximal eigenvalue:

What this means is, for example, that the second elementary subsequence (11) occurs φ times as frequently as the fourth elementary subsequence (1211). By weighting the subsequences by the entries in this vector appropriately, we can calculate the limiting ratio of the number of ones to the number of twos as

Download: Transition matrices [plaintext file]

The Binary “Look-and-Say” Sequence

November 7th, 2010

The look-and-say sequence (which I talked about here) is the sequence that you get by starting with the number 1 and constructing the next term in the sequence by “reading” the previous term. So 1 becomes “one one”, or 11. That becomes “two ones”, or 21. That becomes “one two, one one”, or 1211, and so on.

In this post, I am going to investigate the related binary version of the sequence, which starts off 1, 11 much like the regular sequence. But then when reading 11, we read it as “two ones”. Since two in binary is 10, the next term in the sequence is 101. When reading that term, we read it as “one one, one zero, one one”, so the next term is 111011. That term is read as “three ones, one zero, two ones”, and since three is 11 in binary and two is 10 in binary, the next term is 11110101, and so on. In this post we will answer two questions in particular about this sequence:

1) On average, how much longer is the (n+1)th term in the sequence than the nth term in the sequence?

2) On average, what is the ratio of the number of ones to the number of zeroes in the sequence?

Non-Interacting Subsequences

Much like the regular look-and-say sequence, we are able to study this sequence by constructing a “basis” of non-interacting subsequences that every term in the binary look-and-say sequence is made up of. Fortunately, constructing such a family of subsequences for the binary version of the look-and-say sequence is much simpler than it is for the decimal version of the sequence – here we only need ten different basic subsequences (whereas we needed 92 different subsequences for the regular look-and-say sequence!). These ten subsequences, and the subsequences they evolve into, are summarized in the following table.

# Subsequence Evolves Into
1 1 (2)
2 11 (3)(1)
3 10 (5)
4 110 (3)(4)
5 1110 (6)
6 11110 (7)(4)
7 100 (9)
8 1100 (3)(8)
9 11100 (10)
10 111100 (7)(8)

So for example, the first term in the sequence, 1, evolves into the subsequence (2), which is 11. That term then evolves into subsequence (3) followed by subsequence (1), or 101. That term then evolves into the subsequence (5) followed by the subsequence (2), or 111011, and so on. The reason that this representation of the sequence is useful is we can use it to describe the evolution of the binary look-and-say sequence entirely within a matrix T. In particular, we let T be the matrix with 1 in its (i,j) entry if the subsequence (i) appears in the evolution rule for subsequence (j), and 0 in its (i,j) entry otherwise:

Now if v is a 10-dimensional vector whose ith entry indicates how many times the subsequence (i) appears in a particular term of the binary look-and-say sequence, it follows that the entries of Tv tell us how many times each subsequence appears in the next term of the binary look-and-say sequence. So it follows from standard theory of linear homogeneous recurrence relations that we can now read off all of the long-term behaviour of the binary look-and-say sequence from the eigenvalues and eigenvectors of T.

Rate of Growth of the Sequence

The asymptotic rate of growth of the number of digits in the terms of the binary look-and-say sequence is simply the magnitude of the largest eigenvalue of the transition matrix T above. Using Maple it is simple to derive this value. If Ln is the number of digits in the nth term of the binary look-and-say sequence, then

This limit is approximately 1.465571, which means that the binary version of this sequence grows much faster than the decimal version of the sequence (recall that the growth rate of the number of digits of the regular look and say sequence is approximately 1.303577). This limit is also the unique real root of the cubic x3 – x2 – 1, which follows from the fact that the characteristic polynomial of T is

Ratio of Number of Ones to Zeroes

If we let Nn denote the number of ones in the nth term of the binary look-and-say sequence, and if we let Zn denote the number of zeroes in the nth term of the sequence, what is

In other words, what is the average ratio of ones to zeroes in this sequence? The following table shows the value of Nn/Zn for n = 3, 4, …, 25, which might give some intuition to the problem:

n Nn/Zn
3 2.000
4 5.000
5 3.000
6 2.000
7 2.000
8 2.000
9 1.786
10 1.762
11 1.742
12 1.717
13 1.691
14 1.690
15 1.680
16 1.676
17 1.672
18 1.671
19 1.669
20 1.668
21 1.667
22 1.667
23 1.666
24 1.666
25 1.666

Based on numerical estimates like those given in the table above, it has been conjectured that the limiting ratio is 5/3 (or some nearby value). We will now show that the limit does indeed exist, but its value is not 5/3 — it just happens to be really close to 5/3.

Much like the maximal eigenvalue of T tells us the overall growth rate of the sequence, the corresponding eigenvector tells us the distribution of the different subsequences that are present in the limit. Once we know the distribution of the individual subsequences, it is not difficult to find out the overall ratio of ones to zeroes by weighing the different subsequences appropriately. So our first step is to find the eigenvector corresponding to the maximal eigenvalue. To this end, it will be convenient to let

α is the same as in the previous section, and β is exactly the growth rate limit that we computed. Then the eigenvector corresponding to the maximal eigenvalue of T is:

What this means is that, in the limit, the fifth subsequence, 1110, is β times as frequently-occurring as the sixth subsequence, 11110 (for example). Now we just weigh each subsequence according to how many zeros and ones they contain, and we find the limiting ratio of ones to zeroes is

In particular, this ratio does not equal 5/3, but rather its decimal expansion begins 1.6657272222676… (which is less than 1/1000 away from 5/3).

Edit (September 14, 2022): Standupmaths has made a nice video about this same topic here.

A Derivation of Conway’s Degree-71 “Look-and-Say” Polynomial

October 31st, 2010

The look-and-say sequence is the sequence of numbers 1, 11, 21, 1211, 111221, 312211, …, in which each term is constructed by “reading” the previous term in the sequence. For example, the term 1 is read as “one 1”, which becomes the next term: 11. Then 11 is read as “two ones”, which becomes the next term: 21, and so on.

The remarkable thing about this sequence is that even though it seems at first glance to be quite arbitrary and non-mathematical, it has some interesting properties that were unearthed by John Conway. Most notably, he showed that the number of digits in each term of the sequence on average grows by about 30% from one term to the next. A bit more specifically, he showed that if Ln is the number of digits in the nth term in the sequence, then

where λ is the unique positive real root of the following degree-71 polynomial:

In order to demystify this seemingly bizarre fact, in this post we will show where this polynomial comes from and prove that the above limit does indeed equal its largest root (which happens to be its one and only positive real root).

The Cosmological Theorem

What lets us formally study the look-and-say sequence is a rather ominous-sounding result known as the cosmological theorem, which says that the eighth term and every term after it in the sequence is made up of one or more of 92 “basic” non-interacting subsequences. These 92 basic subsequences are summarized in lexicographical order in the following table. The fourth column in the table says what other subsequence(s) the given subsequence evolves into. For example, the first subsequence, 1112, evolves into the 63rd subsequence: 3112. Similarly, the second subsequence, 1112133, evolves into the 64th subsequence followed by the 62nd subsequence: 31121123.

# Subsequence Length Evolves Into
1 1112 4 (63)
2 1112133 7 (64)(62)
3 111213322112 12 (65)
4 111213322113 12 (66)
5 1113 4 (68)
6 11131 5 (69)
7 111311222112 12 (84)(55)
8 111312 6 (70)
9 11131221 8 (71)
10 1113122112 10 (76)
11 1113122113 10 (77)
12 11131221131112 14 (82)
13 111312211312 12 (78)
14 11131221131211 14 (79)
15 111312211312113211 18 (80)
16 111312211312113221133211322112211213322112 42 (81)(29)(91)
17 111312211312113221133211322112211213322113 42 (81)(29)(90)
18 11131221131211322113322112 26 (81)(30)
19 11131221133112 14 (75)(29)(92)
20 1113122113322113111221131221 28 (75)(32)
21 11131221222112 14 (72)
22 111312212221121123222112 24 (73)
23 111312212221121123222113 24 (74)
24 11132 5 (83)
25 1113222 7 (86)
26 1113222112 10 (87)
27 1113222113 10 (88)
28 11133112 8 (89)(92)
29 12 2 (1)
30 123222112 9 (3)
31 123222113 9 (4)
32 12322211331222113112211 23 (2)(61)(29)(85)
33 13 2 (5)
34 131112 6 (28)
35 13112221133211322112211213322112 32 (24)(33)(61)(29)(91)
36 13112221133211322112211213322113 32 (24)(33)(61)(29)(90)
37 13122112 8 (7)
38 132 3 (8)
39 13211 5 (9)
40 132112 6 (10)
41 1321122112 10 (21)
42 132112211213322112 18 (22)
43 132112211213322113 18 (23)
44 132113 6 (11)
45 1321131112 10 (19)
46 13211312 8 (12)
47 1321132 7 (13)
48 13211321 8 (14)
49 132113212221 12 (15)
50 13211321222113222112 20 (18)
51 1321132122211322212221121123222112 34 (16)
52 1321132122211322212221121123222113 34 (17)
53 13211322211312113211 20 (20)
54 1321133112 10 (6)(61)(29)(92)
55 1322112 7 (26)
56 1322113 7 (27)
57 13221133112 11 (25)(29)(92)
58 1322113312211 13 (25)(29)(67)
59 132211331222113112211 21 (25)(29)(85)
60 13221133122211332 17 (25)(29)(68)(61)(29)(89)
61 22 2 (61)
62 3 1 (33)
63 3112 4 (40)
64 3112112 7 (41)
65 31121123222112 14 (42)
66 31121123222113 14 (43)
67 3112221 7 (38)(39)
68 3113 4 (44)
69 311311 6 (48)
70 31131112 8 (54)
71 3113112211 10 (49)
72 3113112211322112 16 (50)
73 3113112211322112211213322112 28 (51)
74 3113112211322112211213322113 28 (52)
75 311311222 9 (47)(38)
76 311311222112 12 (47)(55)
77 311311222113 12 (47)(56)
78 3113112221131112 16 (47)(57)
79 311311222113111221 18 (47)(58)
80 311311222113111221131221 24 (47)(59)
81 31131122211311122113222 23 (47)(60)
82 3113112221133112 16 (47)(33)(61)(29)(92)
83 311312 6 (45)
84 31132 5 (46)
85 311322113212221 15 (53)
86 311332 6 (38)(29)(89)
87 3113322112 10 (38)(30)
88 3113322113 10 (38)(31)
89 312 3 (34)
90 312211322212221121123222113 27 (36)
91 312211322212221121123222112 27 (35)
92 32112 5 (37)

The important thing about this particular basis of subsequences is that the evolution of any sequence made up of these subsequences is determined entirely by the evolution rule for the subsequences given in the final column of the above table. For example, the eighth term in the look-and-say sequence is 1113213211 = (24)(39). The subsequence (24) evolves into (83) and the subsequence (39) evolves into (9), so the ninth term in the look-and-say sequence is (83)(9), which is 31131211131221.

Computing the Number of Digits in Sequences

Since the evolution of every term in the look-and-say sequence after the eighth can be computed using the table above, we can easily compute the length of every term after the eighth as well. For example, the eighth term in the sequence evolves into (83)(9), so the number of digits of the ninth term in the sequence is 6 + 8 = 14. The subsequence (83) evolves into a subsequence with 10 digits, and (9) evolves into a subsequence with 10 digits, so the tenth term in the look-and-say sequence has 10 + 10 = 20 digits.

All of the information about how the lengths of the 92 subsequences change can be represented in a 92×92 matrix T. In particular, the matrix T has its (i,j) entry equal to Cij × ℓi/ℓj, where Cij is the number of times subsequence (i) appears in the evolution rule for subsequence (j) and ℓi is the length of subsequence (i). This matrix is represented in the following image – white squares represent zero entries in the matrix, and black squares represent the number 2, which is the largest value present in the matrix. Shades of grey represent non-zero numbers, with larger numbers being darker.

Then if we represent a term in the look-and-say sequence as a vector v with its ith entry being ci × ℓi, where ci is the number of times the subsequence (i) appears in that term, we find that the sum of the entries in v is the total length of that term of the look-and-say sequence. More important, however, is the fact that the sum of the entries in Tv is the length of the next term in the look-and-say sequence. The sum of the entries in T2v is the length of the next term in the look-and-say sequence, and so on. So we have found a degree-92 recurrence relation for the length of terms in the look-and-say sequence, and the corresponding transition matrix is T.

Computing the Limit

It is a basic fact of linear homogeneous recurrence relations that a closed-form solution to the recurrence relation can be written down in terms of the eigenvalues of the transition matrix (see the linked Wikipedia page for specifics). As a corollary of this, the limiting ratio of terms in the sequence is equal to the spectral radius of the transition matrix. Fortunately, the transition matrix in this case is quite sparse, so its characteristic polynomial isn’t too difficult to compute:

Indeed, the degree-71 polynomial that λ is a root of is one of the factors of the characteristic polynomial of the transition matrix T. All that remains to do is to get MATLAB to compute the largest root of that polynomial (i.e., the spectral radius of T):

>> max(abs(eig(T)))

ans =

The matrix T is attached below for those who would like to play with it. Something fun to think about: what do the rational eigenvalues (-1, 0, and 1) of T represent?

Download: Transition matrix [plaintext file]

Update [March 17, 2013]: Entry 91 of the subsequence table has been corrected – thanks to Marcus Stuhr and liuguangxi for the correction.

Generating Sequences of Primes in Conway's Game of Life

August 28th, 2009

One of the most interesting patterns that has ever been constructed in Conway’s Game of Life is primer, a gun that fires lightweight spaceships that represent exactly the prime numbers. It was constructed by Dean Hickerson way back in 1991, yet arguably no pattern since then has been constructed that’s as interesting. It seems somewhat counter-intuitive at first that the prime numbers, which seem somehow “random” or “unpredictable”, can be generated by this (relatively simple) pattern in the completely deterministic Game of Life.

Primer, the prime-generating gun

Primer, the prime-generating gun

The gun works by firing lightweight spaceships westward, and destroying them via glider guns that emulate the Sieve of Eratosthenes. A lightweight spaceship makes it past the left edge of the gun at generation 120N if and only if N is a prime number (though for technical reasons, 2 and 3 are not outputted).

The first six lightweight spaceships output by primer and the numbers they represent

The first six lightweight spaceships output by primer

It wasn’t too long after making primer that Hickerson realized that he could attach a gun to the bottom-left corner of it to turn it into a twin prime calculator by allowing each lightweight spaceship through only if another lightweight spaceship passed through 240 generations earlier. Similarly, Jason Summers constructed a Fermat prime calculator in 2000 by shooting a glider at the lightweight spaceship stream every generation of the form 120(2N + 1), which ends up detecting exactly the Fermat primes.

So what other families of primes can we compute in Life by altering the output of the original prime-generating gun?

Mersenne Primes

Mersenne primes can easily be computed using the exact same method as was used in the Fermat prime calculator — use a 7-engine Cordership (in blue below) to bounce a glider back at the stream of lightweight spaceships, with the time required for the glider to reach the stream doubling each time. An inverter (in green below) eliminates all lightweight spaceships that try to get past it unless it just received a glider from the Cordership. By fiddling around with timing a tiny bit, we then have a Mersenne prime calculator:

Mersenne Prime Calculator

Mersenne Prime Calculator

Prime Quadruplets

Four prime numbers are said to form a prime quadruplet if they are of the form (p, p+2, p+6, p+8) for some prime number p, which is the closest that four prime numbers can be together (except for the degenerate cases of (2,3,5,7) and (3,5,7,11)). Prime quadruplets are easy to compute because they can be thought of as consecutive pairs of twin primes. Since we already have a twin prime calculator, we can just repeat its reaction.

The twin prime calculator works by attaching a period 240 gun (in green below) to the bottom-left corner of primer. If it is timed correctly, it has the effect of allowing a lightweight spaceship through at generation 240N if and only if a lightweight spaceship tried to pass through at generation 240(N-1). Thus, it will only allow a lightweight spaceship through if it represents a prime number of the form p+2, where p is another prime number. Well, simply attaching a period 720 gun (in blue below) then allows a spaceship through at generation 720N if and only if a lightweight spaceship tried to pass through at generation 720(N-1). This has the effect of allowing a lightweight spaceship to pass through only if it represents a twin prime pair (p,p+2), and there is another twin prime pair of the form (p-6,p-4). That is, the only lightweight spaceships allowed through are those representing the upper members of prime quadruplets.

Prime quadruplet calculator

Prime quadruplet calculator

Prime Pairs of the Form (p, p+2k)

The twin prime calculator mentioned earlier gives a way of computing prime pairs of the form (p,p+2), but what about pairs where the gap is larger than 2? For example, the k=2 case gives what are known as cousin primes, and the k=3 case gives sexy primes (yes, really).

For the case of cousin primes, the thing to notice is that every pair of cousin primes (except for the first pair, (3,7)) must be of the form (6n+1, 6n+5) for some natural number n. Thus, we can use two period 720 guns (in blue below) to allow only the upper prime in a cousin prime pair to pass through. This is achieved by having the top gun fire at the lightweight spaceships representing primes of the form 6n+1 — if a lightweight spaceship is hit, then a block is created in the path of the other gun, which is fired at lightweight spaceships representing primes of the form 6n+5. If a prime was present at 6n+1, then the lightweight spaceship makes it through unharmed at 6n+5. If there was no prime present at 6n+1, then the bottom gun destroys the lightweight spaceship representing 6n+5.

Cousin prime calculator

Cousin prime calculator

Extending this idea to prime pairs of the form (p,p+2k) for k ≥ 3 is a bit more challenging, however, because it is possible for pairs to overlap. For example, (37,43) is a sexy prime pair, as is (41,47). Up until now we have only been able to detect single pairs at a time, since the block that acts as our “counter” that keeps track of whether a prime was detected earlier is placed in the stream of incoming lightweight spaceships. Thus, if it’s possible for two pairs to overlap, we will get lightweight spaceships colliding with the block, causing a mess.

To get around this problem, we use a device (known as a fanout, in green below) that duplicates the stream of lightweight spaceships. We then check for certain pairs on one stream, and the rest of the pairs on the other stream (these devices are outlined in blue below). Once we’re done, we merge the resulting streams of lightweight spaceships back together (using the devices in purple below).

To make this process a bit more explicit, I present a gun that computes prime pairs of the form (p,p+8). In particular, a lightweight spaceship will make it past the left edge of this pattern at about generation 1620+120N if and only if both N and N+8 are prime.

(p, p+8) prime calculator

(p, p+8) prime calculator

We now have all of the tools needed to build any pattern that computes prime pairs of the form (p, p+2k) as long as k = 1 or 2 (mod 3), though we may need to use the fanout device multiple times if it’s possible for more than one pair to overlap. If k = 0 (mod 3), however, it’s much more difficult to construct the desired pattern, because not only can you have overlapping prime pairs like (5, 11) and (7, 13), but you can have prime pairs in sequence such as (5, 11) and (11, 17). This problem can be remedied using the same tools as used in the (p,p+8) prime calculator, though you may need to use a lot of fanout devices to make things work. For example, computing the sexy primes using these tools would require at least four fanouts, and some clever elimination logic on each of the resulting five lightweight spaceship streams. I don’t feel up to that task myself, but it’s nice to know that we have a method for constructing a sexy prime calculator.

11630 is the First Uninteresting Number

June 12th, 2009

There’s an old math paradox that says that all natural numbers are interesting, since otherwise there would have to be a smallest uninteresting number, and that in itself is pretty interesting. Of course, this is meant to show that ideas in the English language do not always translate to well-defined mathematical concepts, but let’s ignore our better mathematical sense and tinker with the idea of how interesting different numbers are a little bit. In particular, I claim that 11630 is utterly bland and uninteresting.

Why 11630?

Before saying why 11630 is uninteresting, I should probably say what I consider “interesting” to even mean. Interesting, to me, means that it has some (semi-unique) mathematical property that sets it apart from other numbers. 11 is interesting because it is prime, 16 is interesting because it is a perfect power (16 = 42), and so on. Clearly, there is some ambiguity in this definition, since one could consider composite numbers interesting, just as I considered prime numbers interesting. Additionally, do we consider 2719 interesting simply because it is prime? I’d say no, since there are hundreds of prime numbers that come before it — perhaps only the first few numbers that satisfy a given property should be considered interesting as a result of it?

Using these ideas, it seems like determining how interesting a number is would be a task perfectly suited to the Online Encyclopedia of Integer sequences (OEIS). If you’re unfamiliar with it (i.e., if you’re not a math person and have no place reading my blog), the OEIS is a database containing thousands of (you guessed it) integer sequences that have been submitted by users over the last decade or so (such as the sequence of prime numbers 2, 3, 5, 7, 11,… and the sequence of perfect powers 1, 4, 8, 9, 16,…). Presumably, if an integer is interesting then it will appear in at least one or two of the 159437 sequences contained in the database, right? Indeed, it seems that we can get a rough idea of how interesting a number is by looking at how many sequences that number appears in in the database compared to other numbers of similar size.

11630 is the first number that is not listed in a single sequence in the OEIS. It is not prime, nor is it highly composite (11630 = 2×5×1163). It doesn’t have any particularly notable residue properties, and it doesn’t come up in counting problems. It’s boring in every way, and it seems as though not a single mathematician has found a use for it in the last dozen or so years (let me know if you’ve discovered otherwise).

What Numbers are Interesting?

First off, I’m not going to deal with particularly small numbers (say in the range of 1 – 50) since, as the strong law of small numbers quips, these numbers will appear all over the place just because they’re small. You could probably argue that most (if not all) of them are interesting, so I’ll instead take a look at a couple larger numbers that are particularly interesting.

The number 421 appears in some 1894 sequences, while most numbers that size appear in about 940 sequences. This seems to indicate that 421 is a particularly interesting number, but why? What’s so special about 421? Well, it’s prime (in fact, it’s a twin prime, Pythagorean prime, cuban prime, lucky number of Euler prime, additive prime, and irregular prime), it’s congruent to 1 mod 2,3,4, 5, 6,7, 10, 12, it’s the sum of five primes, and 4212 = 4202 + 292. Similarly, 512 appears in 2116 sequences even though most numbers around 512 appear in about 800 sequences. This is perhaps less surprising than 421, since 512 = 29 = 83 = 162 + 162 is a number that somehow seems “nice” due to it being a perfect power. Additionally, 512 is a Leyland number, Harshad number, and it comes up in all sorts of counting problems.

What of the Paradox?

Recalling the paradox from earlier, we are now forced to ask ourselves whether or not 11630 is now interesting as a result of it being the first number not included in the OEIS. Rather than come up with an answer, I’m going to take the easy way out and let the OEIS decide. The sequence of uninteresting numbers is 11630, 12067, 12407, 12887, 13258, 13794, 13882, 13982, 14018, 14163,… Let’s submit that to the OEIS and see if they consider it to be interesting or not.

Update [June 13, 2009]: I got word back via e-mail today that this sequence didn’t make the cut. So there you have it — these numbers truly are uninteresting.

Update [November 12, 2009]: It looks like 11630 is now listed in the OEIS. Additionally, 12067 was recently added, meaning that 12407 is now the first uninteresting number.

Update [October 7, 2011]: Interested readers might want to check out this paper, which explores similar questions and mentions the numbers computed here.

Update [November 14, 2011]: The British television show QI recently aired a segment on exactly this topic. See the video here.

Update [November 22, 2013]: A whole bunch of these numbers have been added to the OEIS lately, making 14228 the new first uninteresting number.


Rectangular Oscillators in the 2×2 (B36/S125) Cellular Automaton

May 22nd, 2009

One interesting Life-like cellular automaton, known as “2×2“, is particularly abundant with frequently-occurring oscillators and has thus attracted some attention from Life enthusiasts over the years. The 2×2 automaton takes place on a grid much like Conway’s Game of Life, but cells are born if they have exactly 3 or 6 neighbours, and they survive if they have exactly 1, 2, or 5 neighbours (this behaviour is summarized by the rulestring “B36/S125”).

Some small oscillators in B36/S125

Some small period 2 oscillators in B36/S125

Take a look at the second oscillator from the left in the top row of the above image — it is a 2 × 4 rectangle that oscillates back and forth. The oscillators that I will be looking at in this post are rectangles like this; rectangles of sizes 2m × 2n such that m + n is odd (the reason for these restrictions on the sizes of the rectangles is simply that they aren’t oscillators otherwise (not quite, see the correction in the comments)). The next two smallest of these rectangular oscillators are shown here:

Block oscillators in B36/S125

Rectangular oscillators in B36/S125

As you can see, the oscillators behave a bit more unpredictably as their size increases. The question that I will answer now is “What is the period of a 2m × 2n rectangular oscillator in the 2×2 rule in terms of m and n?”

I will present the final answer right now, since it isn’t particularly simple: the period is 2k+1 – 2, where k is the least natural number such that 2k = ± 1 mod (m + n). While this answer isn’t exactly pretty, the k values have at least already been computed up to m + n = 2001 (see Sloane’s A003558), and there is at least some theory developed about them (see Suborder Function at Mathworld).

It is perhaps worth noting that B36/S125 isn’t the only rule in which these oscillators work; it’s simply the most well-known. These rectangular oscillators behave the same in any rule from B3/S5 to B3678/S012567.

Deriving the Answer

Disclaimer: The following contains much handwaving. It can be made rigorous, but I don’t particularly want to; the argument is long enough as it is, and this is recreational mathematics, after all.

Before proceeding, let’s make a simplification by noticing that a 2m × 2n rectangle is simply another phase of a 2 × 2(m + n – 1) rectangular oscillator. It is thus enough to consider 2 × 4n rectangular oscillators, where n is an integer.

The thing to notice about these oscillators is that, as described by Dean Hickerson in this thread, each phase of these block oscillators can be described as an XOR of rectangles. For example, consider the following phase of one of the above oscillators:

2 × 12 XOR 6 × 8

2 × 12 XOR 6 × 8

The above phase can be considered the XOR of a 2 × 12 rectangle and a 6 × 8 rectangle. Different phases may require a different number of rectangles to be XORed, but every phase can always be represented in this way. More important, however, is the fact that the evolution of the oscillators occurs in a very predictable way when modeled like this. Consider the following grid. (Aside: you will likely recognize that it resembles very closely one half of the Sierpinski triangle. This is no coincidence; it turns out that these oscillators are, in a sense, emulating the “Rule 90” 1D cellular automaton.)

XOR table

XOR grid

The way to read the above grid is that that row represents the current generation and the column represents the size of the rectangle that is being XORed (the first column represents a 2 × 4n rectangle, the second column represents a 4 × (4n – 2) rectangle, the third column represents a 6 × (4n – 4) rectangle, and so on). Thus, you “start” in the top-left cell, and that represents the 2 × 4n rectangle that you start with (in generation 0). To go to generation 1, go to the next row, where we see that the only filled in cell is in the second column, which is the 4 × (4n – 2) column. Thus, the first generation will just be a filled-in 4 × (4n – 2) rectangle. To see what generation 2 will look like, go to the next row, where we see that two cells are filled in, corresponding to 2 × 4n and 6 × (4n – 4) rectangles. Thus, XOR together two rectangles of those sizes (in the sense described earlier) to get what generation 2 must look like.

The key to determining the oscillators’ periods comes from realizing that if we continue to label the columns in the way I described, eventually we hit zero-length rectangles, which doesn’t make a whole lot of sense. Thus, we simply do not XOR any of those rectangles that are of length zero. But what about rectangles of negative length? Instead of counting down into negative numbers, start counting back up. This is probably easiest to illustrate with an example, so I’ll use the n = 3 case.

Coloured XOR grid (n = 3)

Coloured XOR grid (n = 3)

Red columns represent 2 × 12, orange represent 4 × 10, yellow represent 6 × 8, green represent 8 × 6, blue represent 10 × 4, purple represent 12 × 2, and grey represent zero-width rectangles that can be ignored. Thus, we see that (for example), in generation 9 (row 10), there is a puple and a green and so the oscillator will look like a 12 × 2 rectangle XORed with an 8 × 6 rectangle.

The period can be determined by grids of this type by going down the rows, looking for the first row (after row 1) that has an odd number of red cells filled in (so that after XORing, a 2 × 4n rectangle will be present) and an even number of each other colour besides grey (so that after XORing, no rectangles of other sizes will be present). It can be shown that the first time this occurs must be at the bottom of one the triangles that reaches all the way to the left (so in row 3, 7, 15, 31, and so on). This corresponds to the oscillators always having period that is equal to 2k – 2, where k is some integer. To determine what that integer is, notice that if one of these oscillators starts off as a 2 × 4n rectangle, then it will appear rotated 90 degrees as a 4n × 2 rectangle halfway through its period. Thus, we need the least k such that row 2k-1 contains a single purple cell (i.e., 2k-1 = ± 1 mod (2n + 1)).

By transforming back to 2m × 2n rectangles and shifting around k a little bit, we see that the period is 2k+1 – 2, where k is the least integer such that 2k = ± 1 mod (m + n).

Thus, using Sloane’s A003558 as our guide, we see that the period of a 2m × 2n rectangle for m + n = 3, 5, 7, 9, … is 2, 6, 14, 14, 62, 126, 30, 30, 1022, …

Update [May 28, 2009]: This sequence is now listed in the OEIS as Sloane’s A160657.

Update [January 14, 2019]: A few people (at least Dave Greene as well as wwei and Blinkerspawn in the comments below) noticed that this argument is incorrect — I have just demonstrated an upper bound on the period of these oscillators, and the true period may be a factor of that upper bound (this first happens for the 2-by-72 oscillator, which has period equal to 1/3 of the value claimed in this post). Sequence A160657 has now been updated with the correct values.