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 A118813 #13 Feb 17 2014 06:12:51 %S A118813 3628679,87178286159,20922789847679,265252859812191058635000805631999 %N A118813 Primes of the form (2n)! - n! - 1. %C A118813 The term a(7) (2407 digits) is too large to include even in the b-file. %C A118813 All the numbers end in 9. %C A118813 For all n, (2n)! - n! - 1 has exactly A027868(n) trailing 9s. - _Rick L. Shepherd_, Feb 16 2014 %C A118813 Generated by n = 5, 7, 8, 15, 103, 179, .... - _R. J. Mathar_, Sep 20 2012 (now A238011) %D A118813 G. Balzarotti and P. P. Lava, Le sequenze di numeri interi, Hoepli, 2008, p. 159. %H A118813 Rick L. Shepherd, <a href="/A118813/b118813.txt">Table of n, a(n) for n = 1..6</a> %e A118813 For n=5, (2*5)! - 5! - 1 = 3628800 - 120 - 1 = 3628679, which is prime. %p A118813 PFACT:=proc(N) local i,r; for i from 1 by 1 to N do r:=(2*i)!-i!-1; if isprime(r) then print(i); fi; od; end: PFACT(100); %Y A118813 Cf. A238011 (corresponding n), A118812, A237443. %K A118813 nonn %O A118813 1,1 %A A118813 _Paolo P. Lava_ and _Giorgio Balzarotti_, May 23 2006