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 A278504 #6 Nov 24 2016 09:28:06 %S A278504 1,3,2,6,5,10,4,15,8,21,9,28,7,36,12,45,14,55,11,66,13,78,20,91,17, %T A278504 105,16,120,27,136,23,153,19,171,35,190,18,210,22,231,44,253,30,276, %U A278504 26,300,54,325,29,351,24,378,65,406,25,435,34,465,77,496,38,528,37,561,90,595,47,630,43,666,104,703,31,741,33,780,119,820,46 %N A278504 Inverse permutation to A278503. %H A278504 Antti Karttunen, <a href="/A278504/b278504.txt">Table of n, a(n) for n = 1..10000</a> %H A278504 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %F A278504 a(n) = (1/2) * ((c+r)^2 - r - 3*c + 2), where c = A278538(n), and r = A278539(n). %o A278504 (Scheme) (define (A278504 n) (let ((row (A278539 n)) (col (A278538 n))) (* (/ 1 2) (- (expt (+ row col) 2) row col col col -2)))) %Y A278504 Inverse: A278503. %Y A278504 Cf. A278538, A278539. %Y A278504 Cf. also A278506, A278512. %K A278504 nonn %O A278504 1,2 %A A278504 _Antti Karttunen_, Nov 23 2016