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 A111602 #8 Aug 11 2025 14:00:52
%S A111602 1,6,42,300,2475,22050,220500,2381400,28279125,360186750,4970577150,
%T A111602 73045872900,1150472498175,19174541636250,339663308985000,
%U A111602 6333077180430000,124682456989715625,2572020969902133750
%N A111602 Third column (m=2) of unsigned triangle A111595.
%F A111602 E.g.f.: (1/sqrt(1-x^2))*((x/(1-x))^2)/2!.
%F A111602 a(n) = (n!/2!)*Sum_{k=0..floor((n-2)/2)} binomial(2*k, k)*(n-2*k-1)/(4^k), n >= 2.
%F A111602 D-finite with recurrence (-n+2)*a(n) +2*n*a(n-1) +n*(n-1)^2*a(n-2)=0. - _R. J. Mathar_, Aug 11 2025
%K A111602 nonn,easy
%O A111602 2,2
%A A111602 _Wolfdieter Lang_, Aug 23 2005