This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A306721 #19 Jun 21 2021 03:01:16
%S A306721 1,1,1,1,1,1,1,1,2,9,37,121,331,793,1717,3433,6437,11456,19569,32505,
%T A306721 53449,89149,155041,286825,564929,1163317,2442210,5117225,10558381,
%U A306721 21308121,41973391,80778601,152344397,282855561,520060249,953217792,1753553441,3256528177,6127896977,11694334137
%N A306721 a(n) = Sum_{k=0..n} binomial(k, 7*(n-k)).
%H A306721 Seiichi Manyama, <a href="/A306721/b306721.txt">Table of n, a(n) for n = 0..3556</a>
%H A306721 <a href="/index/Rec#order_08">Index entries for linear recurrences with constant coefficients</a>, signature (7,-21,35,-35,21,-7,1,1).
%F A306721 G.f.: (1-x)^6/((1-x)^7-x^8).
%F A306721 a(n) = 7*a(n-1)-21*a(n-2)+35*a(n-3)-35*a(n-4)+21*a(n-5)-7*a(n-6)+a(n-7)+a(n-8) for n > 7.
%F A306721 a(n) = A017857(7*n).
%t A306721 a[n_] := Sum[Binomial[k, 7*(n-k)], {k, 0, n}]; Array[a, 40, 0] (* _Amiram Eldar_, Jun 21 2021 *)
%o A306721 (PARI) {a(n) = sum(k=0, n, binomial(k, 7*(n-k)))}
%o A306721 (PARI) N=66; x='x+O('x^N); Vec((1-x)^6/((1-x)^7-x^8))
%Y A306721 Column 7 of A306680.
%Y A306721 Cf. A017857.
%K A306721 nonn,easy
%O A306721 0,9
%A A306721 _Seiichi Manyama_, Mar 06 2019