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 A381281 #8 Feb 19 2025 03:38:05
%S A381281 1,1,2,33,240,2145,33120,480753,7878528,158696577,3384322560,
%T A381281 78934776129,2053186983936,57231998680545,1714372871178240,
%U A381281 55323775198258065,1899762412262031360,69264871449203672577,2677542944055160209408,109197154520146527569505
%N A381281 Expansion of e.g.f. 1/(1 - x * cosh(3*x)).
%C A381281 As stated in the comment of A185951, A185951(n,0) = 0^n.
%F A381281 a(0) = 1; a(n) = Sum_{k=0..floor((n-1)/2)} 9^k * (2*k+1) * binomial(n,2*k+1) * a(n-2*k-1).
%F A381281 a(n) = Sum_{k=0..n} k! * 3^(n-k) * A185951(n,k).
%o A381281 (PARI) a185951(n, k) = binomial(n, k)/2^k*sum(j=0, k, (2*j-k)^(n-k)*binomial(k, j));
%o A381281 a(n) = sum(k=0, n, k!*3^(n-k)*a185951(n, k));
%Y A381281 Cf. A205571, A381280.
%Y A381281 Cf. A185951.
%K A381281 nonn
%O A381281 0,3
%A A381281 _Seiichi Manyama_, Feb 18 2025