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 A138523 #39 Aug 25 2024 17:01:28
%S A138523 1,7,127,5167,368047,40284847,6267305647,1313941673647,
%T A138523 357001369769647,122002101778601647,51212944273488041647,
%U A138523 25903229683158464681647,15537113273014144448681647,10904406563691366305216681647,8852666400303393320848832681647,8231691320578226211046411712681647
%N A138523 a(n) = Sum_{k=1..n} (2k-1)!.
%C A138523 a(n) is divisible by 107 for n >= 53. - _Robert Israel_, Dec 01 2015
%C A138523 Last digit is 7 for n > 1. Therefore there is no square in this sequence except 1. - _Altug Alkan_, Dec 01 2015
%C A138523 a(n) is the rank of [2,1, 4,3, ..., 2n,2n-1] within the permutations of [1, 2, ... 2n-1, 2n] in lexicographic order. See A375302 for the ranking function. - _Hugo Pfoertner_, Aug 25 2024
%H A138523 G. C. Greubel, <a href="/A138523/b138523.txt">Table of n, a(n) for n = 1..225</a>
%F A138523 Recurrence: a(1) = 1, a(2) = 7, a(n) = (4*n^2-6*n+3)*a(n-1) - 2*(n-1)*(2*n-1)*a(n-2). - _Vladimir Reshetnikov_, Oct 28 2015
%p A138523 a:=proc(n) options operator, arrow: sum(factorial(2*k-1), k=1..n) end proc: seq(a(n), n=1..14); # _Emeric Deutsch_, Mar 31 2008
%t A138523 Table[Sum[(2i - 1)!, {i, n}], {n, 15}] (* _Stefan Steinerberger_, Mar 25 2008 *)
%o A138523 (PARI) a(n) = sum(k=1, n, (2*k-1)!); \\ _Michel Marcus_, Oct 28 2015
%Y A138523 Cf. A007489, A138524, A138525, A375302.
%K A138523 easy,nonn
%O A138523 1,2
%A A138523 _Leroy Quet_, Mar 23 2008
%E A138523 More terms from _Stefan Steinerberger_, _Emeric Deutsch_ and _Robert G. Wilson v_, Mar 25 2008