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 A131391 #5 Nov 29 2017 03:34:09 %S A131391 1,2,4,5,8,10,11,13,16,17,19,21,24,25,28,29,32,33,35,37,39,41,44,46, %T A131391 47,49,51,53,56,57,59,61,63,65,68,69,72,73,75,77,79,81,83,85,88,90,92, %U A131391 94,95,97,99,101,103,105,108,109,111,113,115,117,119,121,123,125,127,129 %N A131391 Positions of positive integers in A131389, minus 1. %C A131391 If A131389 is a permutation of the integers, as conjectured, then A131391 is the complement of A131392. %F A131391 a(n) = -1 + position of the n-th positive integer in the sequence A131389=(0,1,2,-1,3,4,-2,-3,6,-4,5,...) %e A131391 The 5th positive integer to occur in A131391 is 6, which occurs in position 9, so that a(5) = 9 - 1 = 8. %Y A131391 Cf. A131388, A131389, A131390, A131392, A131393, A131394, A131395, A131396, A131397. %K A131391 nonn %O A131391 1,2 %A A131391 _Clark Kimberling_, Jul 06 2007