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 A186720 #19 Mar 29 2025 18:14:14
%S A186720 1,4,144,3600,1270080,153679680,519437318400,150117385017600,
%T A186720 221193371393280,6450247552370862240000,5424658191543895143840000,
%U A186720 20852386088294732932920960000,28546916554875489385168794240000,6855338104106528236638391873920000,12675520154492970709544386574878080000
%N A186720 As p runs through the primes, sequence gives denominator of Sum_{k=1..p-1} 1/k^2.
%D A186720 Paulo Ribenboim, The Little Book of Bigger Primes, Springer-Verlag NY 2004. See pp. 22-23.
%H A186720 R. Mestrovic, <a href="http://arxiv.org/abs/1111.3057">Wolstenholme's theorem: Its Generalizations and Extensions in the last hundred and fifty years (1862-2011)</a>, arXiv:1111.3057 [math.NT], 2011.
%p A186720 f2:=proc(n) local p;
%p A186720 p:=ithprime(n);
%p A186720 denom(add(1/i^2,i=1..p-1));
%p A186720 end proc;
%p A186720 [seq(f2(n),n=1..20)];
%t A186720 a[n_] := HarmonicNumber[Prime[n] - 1, 2] // Denominator;
%t A186720 Array[a, 15] (* _Jean-François Alcover_, Nov 25 2017 *)
%Y A186720 Cf. A125551, A061002.
%K A186720 nonn
%O A186720 1,2
%A A186720 _N. J. A. Sloane_, Jan 21 2012