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 A257506 #7 May 06 2015 21:44:24 %S A257506 1,3,6,2,10,15,21,28,36,45,55,5,66,78,91,9,105,4,120,136,153,14,171, %T A257506 190,210,231,253,276,300,325,351,378,406,435,465,496,528,561,595,630, %U A257506 666,703,741,780,820,861,903,20,946,990,1035,27,1081,1128,1176,1225,1275,1326,1378,35,1431,1485,1540,44,1596,54,1653,1711,1770,65,1830,8 %N A257506 Inverse permutation to A257505. %H A257506 Antti Karttunen, <a href="/A257506/b257506.txt">Table of n, a(n) for n = 1..5050</a> %H A257506 <a href="/index/Fa#facbase">Index entries for sequences related to factorial base representation</a> %H A257506 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %F A257506 a(n) = (1/2) * ((c+r)^2 - r - 3*c + 2), where r = A257681(n), and c = A257679(n). %o A257506 (Scheme) (define (A257506 n) (let ((row (A257681 n)) (col (A257679 n))) (* (/ 1 2) (- (expt (+ row col) 2) row col col col -2)))) %Y A257506 Inverse: A257505. %Y A257506 Cf. A257679, A257681. %K A257506 nonn %O A257506 1,2 %A A257506 _Antti Karttunen_, May 06 2015