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 A278506 #8 Nov 24 2016 09:28:33 %S A278506 1,2,3,4,5,7,6,11,9,16,8,22,10,29,14,37,12,46,15,56,13,67,17,79,20,92, %T A278506 21,106,23,121,27,137,18,154,30,172,19,191,28,211,38,232,35,254,24, %U A278506 277,47,301,36,326,26,352,57,379,25,407,31,436,68,466,44,497,45,529,80,562,54,596,39,631,93,667,34,704,32 %N A278506 Inverse permutation to A278505. %H A278506 Antti Karttunen, <a href="/A278506/b278506.txt">Table of n, a(n) for n = 1..10000</a> %H A278506 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %F A278506 a(n) = (1/2) * ((c+r)^2 - r - 3*c + 2), where c = A278539(n), and r = A278538(n). %o A278506 (Scheme) (define (A278506 n) (let ((row (A278538 n)) (col (A278539 n))) (* (/ 1 2) (- (expt (+ row col) 2) row col col col -2)))) %Y A278506 Inverse: A278505. %Y A278506 Cf. A278538, A278539. %Y A278506 Cf. also A278504, A278512. %K A278506 nonn %O A278506 1,2 %A A278506 _Antti Karttunen_, Nov 23 2016