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 A218219 #8 Feb 06 2018 12:05:04 %S A218219 1,1,1,5,41,436,5741,90481,1663585,34988311,828931049,21851881930, %T A218219 634556225161,20129592507025,692665874901013,25699370092119569, %U A218219 1022757988416562049,43461563755791470416,1964206882303435582865,94076863910519354420761,4760163616791818063701801 %N A218219 Define a(x,y) to be 1 if x is 0 or 1 and y*a(x-1,y)-a(x-2,y) otherwise. Then the n-th term of the sequence is a(n,n). %C A218219 The sequence grows about as fast as x!*e^x/p(x), where p(x) is a quadratic. %e A218219 For 3, by definition, a(0,3)=1 and a(1,3)=1, so a(2,3)=3*a(1,3)-a(0,3)=3-1=2 and a(3,3)=3*a(2,3)-a(1,3)=6-1=5, so the third element is 5 %Y A218219 Cf. A218220. %K A218219 nonn %O A218219 0,4 %A A218219 _Nico Brown_, Oct 23 2012