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 A277881 #8 Nov 05 2016 07:26:47 %S A277881 1,3,2,6,5,4,10,15,9,8,21,7,28,36,14,45,20,13,55,12,66,78,27,11,91, %T A277881 105,35,120,44,19,136,153,54,26,171,18,190,210,65,17,231,253,77,276, %U A277881 90,34,300,16,325,351,104,378,119,43,406,435,135,53,465,25,496,528,152,561,170,64,595,33,630,666,189,24,703,741,209,780,230,76,820,23 %N A277881 Inverse permutation to A277880. %H A277881 Antti Karttunen, <a href="/A277881/b277881.txt">Table of n, a(n) for n = 1..16384</a> %H A277881 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %F A277881 a(n) = (1/2) * ((c+r)^2 - r - 3*c + 2), where c = A277822(n), and r = A277813(n). %o A277881 (Scheme) (define (A277881 n) (let ((row (A277813 n)) (col (A277822 n))) (* (/ 1 2) (- (expt (+ row col) 2) row col col col -2)))) %Y A277881 Inverse: A277880. %Y A277881 Cf. A277813, A277822. %Y A277881 Cf. A028401 (terms at powers of 2). %K A277881 nonn %O A277881 1,2 %A A277881 _Antti Karttunen_, Nov 03 2016