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 A061557 #8 Nov 20 2019 09:10:41
%S A061557 3,8,23,70,222,726,2431,8294,28730,100776,357238,1277788,4605980,
%T A061557 16715250,61020495,223931910,825632610,3056887680,11360977650,
%U A061557 42368413620,158498860260,594636663660,2236748680998,8433988655580
%N A061557 a(n) = (7*n+2)*C(n)/(n+2), where C(n) is the n-th Catalan number.
%C A061557 The identity a(n) = Sum_{k = 0..n} 3*(k-1)*C(k)*C(n-k)/(2*k-1) was verified using the Wilf-Zeilberger theory for hypergeometric sums. The sum arises in the enumeration of separable 1324-avoiding permutations: A026009(n) = a(n)/2 + 2*C(n-1) - 5*C(n)/2.
%C A061557 a(n) = 2*C(n+1) - C(n), with C(n) = A000108(n). - _Ralf Stephan_, Jan 13 2004
%Y A061557 Cf. A000108, A026009.
%Y A061557 A000782 shifted left.
%K A061557 easy,nonn
%O A061557 1,1
%A A061557 Darko Marinov (marinov(AT)lcs.mit.edu), May 17 2001