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 A277821 #5 Nov 05 2016 07:23:20 %S A277821 1,3,2,10,4,5,6,36,9,8,28,14,15,21,7,136,11,27,120,19,91,105,20,44,45, %T A277821 55,13,78,35,12,66,528,77,17,496,90,435,465,43,53,325,351,54,406,18, %U A277821 65,378,152,153,171,16,210,135,34,190,300,26,119,276,25,231,253,104,2080,118,275,2016,32,1891,1953,252,324,1653,1711,33 %N A277821 Inverse permutation to A277820. %H A277821 Antti Karttunen, <a href="/A277821/b277821.txt">Table of n, a(n) for n = 1..16383</a> %H A277821 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %F A277821 a(n) = (1/2) * ((c+r)^2 - r - 3*c + 2), where c = A277818(n), and r = A268671(n). %o A277821 (Scheme) %o A277821 (define (A277821 n) (let ((row (A268671 n)) (col (A277818 n))) (* (/ 1 2) (- (expt (+ row col) 2) row col col col -2)))) %Y A277821 Inverse: A277820. %Y A277821 Cf. A268671, A277818. %K A277821 nonn %O A277821 1,2 %A A277821 _Antti Karttunen_, Nov 03 2016