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 A128648 #12 Feb 16 2025 08:33:05
%S A128648 1,2,4,12,60,5,80,720,7920,55440,55440,6160,6160,18480,425040,5525520,
%T A128648 160240080,160240080,53413360,53413360,480720240,480720240,
%U A128648 19709529840,19709529840,39419059680,197095298400,3350620072800,177582863858400
%N A128648 a(n) = denominator(Sum_{k=1..n} (-1)^(k+1)/(prime(k)-1)).
%C A128648 Numbers m such that a(m) equals A128646(m) are listed in A128649.
%H A128648 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimeSums.html">Prime Sums</a>
%F A128648 a(n) = denominator(Sum_{k=1..n} (-1)^(k+1)/(prime(k)-1)).
%t A128648 Table[Denominator[Sum[(-1)^(k+1)*1/(Prime[k]-1),{k,1,n}]],{n,1,36}]
%Y A128648 Cf. A128647 (numerator(Sum_{k=1..n} (-1)^(k+1)/(prime(k)-1))).
%Y A128648 Cf. A128646 (denominator(Sum_{k=1..n} 1/(prime(k)-1))).
%Y A128648 Cf. A128649, A120271, A119686, A006093, A000040.
%K A128648 frac,nonn
%O A128648 1,2
%A A128648 _Alexander Adamchuk_, Mar 18 2007