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 A004782 #34 Mar 02 2024 00:03:05 %S A004782 2,3,7,16,21,29,43,46,67,78,89,92,105,111,127,141,154,157,171,188,191, %T A004782 205,210,211,221,229,232,239,241,267,277,300,309,313,316,323,326,331, %U A004782 346,369,379,415,421,430,436,441,443,451,460,461,465,469,477 %N A004782 Numbers k such that 2*(2k-3)!/(k!*(k-1)!) is an integer. %C A004782 Superset of A081767, as proved by Luke Pebody. Terms not in A081767 include 3, 7, 127, 511, ... - _Ralf Stephan_, Oct 12 2004 %C A004782 See A260642 for A004782 \ A081767. - _M. F. Hasler_, Nov 11 2015 %C A004782 Equivalently, numbers k such that binomial(2k-3,k-1) == 0 (mod k*(k-1)/2), or: binomial(2k-2,k-1) == 0 (mod k^2-k), or: the Catalan number A000108(k-1) is divisible by k-1, i.e., a(n) = A014847(n) + 1. Indeed, 2(2k-3)!/(k!*(k-1)!) = 2(2k-2)!/(k!(k-1)!(2k-2)) = C(k-1)/(k-1). - _M. F. Hasler_, Nov 11 2015 %H A004782 Vincenzo Librandi, <a href="/A004782/b004782.txt">Table of n, a(n) for n = 1..1000</a> %F A004782 a(n) = A014847(n) + 1. - _Enrique Pérez Herrero_, Feb 03 2013 %t A004782 Select[Range[500], IntegerQ[2 (2 # - 3)!/(#! (# - 1)!)] &] (* _Arkadiusz Wesolowski_, Sep 06 2011 *) %o A004782 (PARI) for(n=2, 999, binomial(2*n-2, n-1)%(n^2-n)||print1(n", ")) %o A004782 (PARI) is_A004782(n)=!binomod(2*n-2, n-1, n^2-n) \\ Using http://home.gwu.edu/~maxal/gpscripts/binomod.gp by M. Alekseyev. - _M. F. Hasler_, Nov 11 2015 %K A004782 nonn %O A004782 1,1 %A A004782 _R. K. Guy_ %E A004782 Offset corrected and initial term added by _Arkadiusz Wesolowski_, Sep 06 2011