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 A265403 #15 Dec 13 2015 07:55:03
%S A265403 10,12,21,22,24,30,34,36,40,51,52,55,57,69,70,76,82,84,87,90,96,99,
%T A265403 100,106,112,114,115,117,120,129,132,136,141,142,147,154,156,159,166,
%U A265403 174,177,180,184,187,192,195,201,205,210,216,217,220,222,225,231,232,234,240,244,246,250,252,255,261,262,274,279,282,285,286,294,297,300
%N A265403 Numbers n for which gcd{k=1..n-1} binomial(2*n, 2*k) = 2n-1.
%H A265403 Antti Karttunen, <a href="/A265403/b265403.txt">Table of n, a(n) for n = 1..2014</a>
%t A265403 Select[Range@ 300, GCD @@ Array[Function[k, Binomial[2 #, 2 k]], {# - 1}] == 2 # - 1 &] (* _Michael De Vlieger_, Dec 11 2015 *)
%o A265403 (PARI) isok(n) = (n>1) && gcd(vector(n-1, k, binomial(2*n, 2*k))) == 2*n-1; \\ _Michel Marcus_, Dec 08 2015, edited by _Antti Karttunen_, Dec 11 2015 (see A265388 for why).
%Y A265403 Cf. A265388.
%Y A265403 Cf. also A265402.
%K A265403 nonn
%O A265403 1,1
%A A265403 _Antti Karttunen_, Dec 08 2015