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 A054873 #8 Feb 26 2022 19:36:49 %S A054873 1,2,6,24,120,720,5040,40320,357840,3427200,34685280,365541120, %T A054873 3971615760,44181889920,500788985760,5763231048960,67163932069920, %U A054873 791030794095360,9400660122813120,112587818898562560 %N A054873 Number of (S_5 67)-avoiding permutations. %H A054873 Elena Barcucci, Alberto Del Lungo, Elisa Pergola, Renzo Pinzani, <a href="https://hal.inria.fr/hal-00958943">Permutations avoiding an increasing number of length-increasing forbidden subsequences</a>, Discrete Mathematics and Theoretical Computer Science 4, 2000, 31-44 %F A054873 f := (x, j)->1-(j+1)*x- sqrt(1-2*(j+1)*x+(j-1)^2*x^2); t := (x, j)->sum(k!*x^k, k=1..(j-1)); s := (x, j)->x^(j-2)*(j-1)!*(f(x, j))/(2)+ t(x, j); j := 6 %Y A054873 Cf. A000108. %K A054873 nonn %O A054873 1,2 %A A054873 Elisa Pergola (elisa(AT)dsi.unifi.it), May 26 2000