[seqfan] Re: A049310 (by way of moderator)
L. Edson Jeffery
lejeffery2 at gmail.com
Fri Sep 2 05:34:12 CEST 2016
Hello Robert,
Thank you for replying to my message (http://list.seqfan.eu/
pipermail/seqfan/2016-August/016685.html). I have a couple of questions.
You wrote:
"If x = 2*(s+1/s), then S(N,x) = Sum_{k=0..N} s^{-N+2k}. In particular, for
x = 2*cos(k*Pi/n) with integer k not divisible by n, s = exp(i*k*Pi/n) is a
2n'th root of unity, and the periodicity follows from that."
For your first statement, is your sum
S(N,x) = Sum_{k=0..N} s^{-N+2k}
missing a coefficient, and could it instead be written as
S(N,x) = Sum_{k=0..N} A(N-k,k)*s^{-N+2k},
where A(N-k,k) is the upward antidiagonal entry in row N-k and column k of
the array A in which row N has generating function (2-z)^N/(1-2z)^(N+1)? It
appears that this array is symmetric and starts as
1 2 4 8 16 32 64 ...
2 7 20 52 128 304 704 ...
4 20 73 230 664 1808 4720 ...
8 52 230 847 2792 8536 24704 ...
16 128 664 2792 10321 34922 110716 ...
32 304 1808 8536 34922 129367 445244 ...
64 704 4720 24704 110716 445244 1651609 ...
...
Row or column 0 is http://oeis.org/A000079 and row or column 1 is
http://oeis.org/A066373, and the array is not in OEIS (not that it should
be). The following Mathematica code (replacing N with n and A with a)
displays the array:
a[n_, k_] := SeriesCoefficient[Series[(2 - z)^n/(1 - 2 z)^(n + 1), {z, 0,
10}], k]; Table[a[n, k], {n, 0, 6}, {k, 0, 6}] // Grid
For your second statement, I understand the connection to 2n-th roots of
unity which you gave (nice, by the way) but, since I have been trying to
work out this proof on my own for a long time without success, could you
please give the details of how the result simply follows from that
connection?
Thank you for your time,
Ed Jeffery
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