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 A087178 #13 Apr 21 2025 02:31:26
%S A087178 1,4,106,630,26185,178666,8991709,3678166249,28031525940,
%T A087178 13143231877116,824038411943548,6585233816779471,427212999940414402,
%U A087178 230740449590738871715,128696758387000610268418,1065763722815793191814814,614675485079469646826034034
%N A087178 ((1/2)/p^2)*(T(p)-1) where p runs through the prime >= 5 and T(k) is the k-th central trinomial number (A002426).
%H A087178 Amiram Eldar, <a href="/A087178/b087178.txt">Table of n, a(n) for n = 1..317</a>
%F A087178 T(n) == 1 (mod n^2) iff n is prime.
%t A087178 f[0] = f[1] = 1; f[n_] := f[n] = ((2*n-1)*f[n-1] + 3*(n-1)*f[n-2])/n; Table[(f[p]-1)/(2*p^2), {p, Prime[Range[3, 20]]}] (* _Amiram Eldar_, Apr 21 2025 *)
%Y A087178 Cf. A002426.
%K A087178 nonn
%O A087178 1,2
%A A087178 _Benoit Cloitre_, Oct 19 2003