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 A355851 #16 Jul 20 2022 22:37:34 %S A355851 0,0,0,1,0,1,0,2,0,2,0,2,2,2,1,2,1,9,0,2,2,4,0,8,0,4,0,2,1,2,7,0,2,2, %T A355851 3,0,5,0,4,4,13,0,2,1,12,0,4,1,15,0,4,7,0,4,4,3,0,3,21,0,4,1,4,6,0,3, %U A355851 2,9,0,5,0,4,2,24,0,2,6,0,4,9,50,0,3,8,0,4,7,21,0,3,17 %N A355851 The difference between the indices of the two most recent instances of a(n-1), not including a(n-1) itself, or 0 if not enough such instances exist. %C A355851 This is similar to Van Eck's sequence: A181391. %e A355851 Say you have 0, 0, 0, 1, 0, 1, 0, 2, 0, 2, 0, 2, 2, ? %e A355851 The previous term is a 2, and the distance between the pair of most recent 2s before that is 2, hence ? = 2. %Y A355851 Cf. A181391. %K A355851 nonn %O A355851 1,8 %A A355851 _Eric Fox_, Jul 19 2022