%I #36 Dec 09 2017 07:07:30

%S 1,6,15,28,42,45,66,77,91,110,126,140,153,156,170,187,190,204,209,210,

%T 220,228,231,238,266,276,299,308,312,315,322,325,330,345,378,414,420,

%U 429,435,440,442,450,459,460,468,476,483,493,496

%N Numbers k such that (2*k - 1)!/(k!)^2 is an integer.

%C Original name was: Numbers n such that gcd(2*n,C(2*n,n))=2*n.

%C For n a prime power (see A000961) we have gcd(2*n,C(2*n,n))=2. - _Arkadiusz Wesolowski_, Jul 01 2012

%C Also, positions where A075055 differs from A000984. - _Ralf Stephan_, Dec 11 2004

%H Giovanni Resta, <a href="/A058008/b058008.txt">Table of n, a(n) for n = 1..10000</a>

%F Appears to be A067348(n)/2. - _Benoit Cloitre_, Mar 21 2003

%F Terms >1 are given by A002503+1. - _Benoit Cloitre_, Dec 09 2017

%t Select[Range[500], IntegerQ[(2 # - 1)!/#!^2] &] (* _Arkadiusz Wesolowski_, Jul 01 2012 *)

%Y Cf. A000961, A001405, A002503, A075055, A000984, A067348.

%K nonn

%O 1,2

%A _Labos Elemer_, Nov 13 2000

%E Name changed by _Arkadiusz Wesolowski_, Jul 01 2012