# [seqfan] wave-like patterns in a conjecture about circular differences

Jon Wild wild at music.mcgill.ca
Tue Dec 18 04:57:27 CET 2018

```Dear sequence fans,

from a list of N numbers, repeatedly take their N successive differences,
including the "wrap-around" difference between last and first entry.
Thwaites reported the conjecture (since proven) that eventually, as long
as N is a power of 2, you'll end up with differences that are all zeroes.
See https://www.futilitycloset.com/2018/11/29/two-puzzles-2/

For some given N, I thought I would see what would be the highest number
of times you'd have to iterate before obtaining all zeroes, for different
maximum entries in the starting list. So for a list of 8 numbers, for
example, the longest chain before hitting all zeroes will have the
following length, dependent on the highest number in the list (determined
empirically by testing all lists):

max entry m  |   longest chain    |    starting list
-------------|--------------------|---------------------
1        |         8          |    {1,0,0,0,0,0,0,0}
2        |         8          |    {2,0,0,0,0,0,0,0}
3        |        14          |    {3,0,0,0,0,0,0,1}
4        |        15          |    {4,0,0,0,0,0,1,2}
5        |        15          |    {5,0,0,0,0,0,3,3}
6        |        15          |    {6,0,0,0,0,0,1,4}
7        |        18          |    {7,0,0,3,6,1,7,0}
8        |        21          |    {8,0,0,0,1,2,4,4}
9        |        21          |    {9,0,0,0,1,4,1,4}
10        |        22          |    {10,0,0,1,3,6,10,5}
11        |        22          |    {11,0,0,0,0,1,5,4}
12        |        22          |    {12,0,0,0,0,2,6,7}
13        |        22          |    {13,0,0,0,0,0,7,11}
14        |        22          |    {14,0,0,0,0,0,0,3}
15        |        22          |    {15,0,0,0,0,1,5,6}
16        |        22          |    {16,0,0,0,0,0,4,7}
17        |        22          |    {17,0,0,0,0,0,2,8}
...

The middle column gives a sequence, and I'll be submitting three
sequences to the OEIS, namely the length of the longest chain as a
function of m, for starting lists of 4, 8, and 16 numbers.

Now, there are lots of other starting lists that produce the maximum
length for each m--the ones shown above just happen to be the earliest
ones my program found that remained unbeaten in length. So I was also
curious about the overall distribution of chain lengths for each value of
m, to see what the range of expected lengths might be, not just the
maximum length. I programmed a sample of millions of randomised lists, I
noted how long each list took to reach all zeroes, and here is where I
found something that looked pretty interesting. I had my program output
bar charts for various values of m, showing the percentage of lists that
survived for each length of iterations before achieving all zeroes. The
pdf file linked below applies to the case N=16, and gathers together
graphs showing the frequency (as a percentage) of each chain-length
required to reach all zeroes, for randomly filled lists of 16 numbers
whose highest value m is shown at the top of each graph:

http://www.music.mcgill.ca/~wild/16numbers_diffToZero_100andUp.pdf

So the first page says that for lists of 16 numbers where the greatest
entry is 100, almost 20% of random lists require exactly 30 iterations of
taking differences before all zeroes are achieved, followed closely by
lists that require 31, then 29 iterations. There are smaller peaks in the
graph, one to the left cresting at 16 iterations, and to the right
cresting somewhere in the mid-40s, and there's a long tail of very low
probabilities stretching off far to the right.

If you can download and display that pdf so it shows a single page at a
time, and if you then flip through the different charts to simulate
animation of the results, you will see a wave-like pattern emerge as m
varies from 100, 200, 500, 1000, 2000, 5000, 10000, 20000, 50000 etc. Or
in fact I just stitched those images together into an animated gif which
you can view here:

https://imgur.com/a/rjzNSdr

That seems like interesting behaviour, no? As you can see the waves are
"standing", in that the peaks and troughs remain in constant position as
the wave washes by, gradually flattening out. Successive troughs are
separated by 15, with minima appearing for lengths of 17, 32, 47, 62,
77...

I left off the lowest values of m in this series of charts, because the
initial crest they form, at a chain length of 16, overwhelms the scale of
these charts. (It starts at 50% when m=1; you can see the tail end of that
crest in the first of the series of charts.)

The behaviour for N=4 looks quite different:

http://www.music.mcgill.ca/~wild/4numbers_diffToZero.pdf

Here the "wave" has a single crest, doesn't go anywhere, and the
distribution settles down and hardly changes between N=500 and
N=5,000,000, with close to 50% (maybe tending to exactly 50%?) of all
lists requiring only 4 iterations to reach zeroes.

The behaviour for N=8 is predictably intermediate between the previous
two cases:

http://www.music.mcgill.ca/~wild/8numbers_DiffsToZero.pdf

Here an early series of waves collapses into an asymmetrical blob and
doesn't change much over the last few increases in order of magnitude,
with an ill-defined "peak" just perceptible at lengths of around 20 or 21.

Not sure what else to do with these observations other than post these
pictures here, as I have no mathematics to share! Seems like it might
interest sequence fans though--does anyone have any insight?

Jon Wild

```