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 A121675 #5 Jun 12 2015 05:47:09
%S A121675 1,1,7,43,371,3926,47622,654151,9999523,167557174,3046387103,
%T A121675 59616689595,1247357472869,27747682830531,653192297754076,
%U A121675 16206706672425167,422358302959175123,11526119161103900834
%N A121675 a(n) = [x^n] (1 + x*(1+x)^(n+1) )^n.
%F A121675 a(n) = Sum_{k=0..n} C(n,k) * C((n+1)*k,n-k).
%e A121675 At n=4, a(4) = [x^4] (1 + x*(1+x)^5 )^4 = 371, since
%e A121675 (1 + x*(1+x)^5 )^4 = 1 + 4*x + 26*x^2 + 104*x^3 + 371*x^4 +...
%t A121675 Table[Sum[Binomial[n,k] * Binomial[(n+1)*k,n-k], {k,0,n}], {n, 0, 20}] (* _Vaclav Kotesovec_, Jun 12 2015 *)
%o A121675 (PARI) a(n)=sum(k=0,n,binomial(n,k)*binomial((n+1)*k,n-k))
%Y A121675 Cf. variants: A121673, A121674, A121676-A121680.
%K A121675 nonn
%O A121675 0,3
%A A121675 _Paul D. Hanna_, Aug 15 2006