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 A123591 #7 Sep 08 2022 08:45:28
%S A123591 -1,0,5,90075,25657845139503479,
%T A123591 516742576223066713590751888575037849059948015
%N A123591 a(n) = ((2^n - 1)^(2^n) - 1)/(2^n)^2.
%C A123591 The next term is too large to include.
%C A123591 Last digit of a(n) is 5 or 9 for n>1. It appears that a(4k) == 4 mod 5 and a(4k+1) == a(4k+2) == a(4k+3) == 0 mod 5.
%C A123591 p divides a(p) for prime p>2. Composite numbers n such that n divides a(n) are listed in A127643 = {15,51,65,85,185,221,255,341,451,533,561,595,645,679,771,...}. - _Alexander Adamchuk_, Jan 22 2007
%H A123591 G. C. Greubel, <a href="/A123591/b123591.txt">Table of n, a(n) for n = 0..8</a>
%F A123591 a(n) = ((2^n - 1)^(2^n) - 1)/(2^n)^2.
%F A123591 a(n) = A085606(2^n)/(2^n)^2.
%t A123591 Table[((2^n-1)^(2^n)-1)/(2^n)^2,{n,0,7}]
%o A123591 (PARI) for(n=0,7, print1(((2^n - 1)^(2^n) - 1)/(2^n)^2, ", ")) \\ _G. C. Greubel_, Oct 26 2017
%o A123591 (Magma) [((2^n - 1)^(2^n) - 1)/(2^n)^2: n in [0..7]]; // _G. C. Greubel_, Oct 26 2017
%Y A123591 Cf. A085606 (n-1)^n - 1.
%Y A123591 Cf. A127643.
%K A123591 sign
%O A123591 0,3
%A A123591 _Alexander Adamchuk_, Nov 13 2006
%E A123591 More terms from _Alexander Adamchuk_, Jan 22 2007