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 A213270 #14 Jun 09 2012 04:54:55 %S A213270 1,2,2,2,4,10,20,18,20,28,36,34,50,46,62,40,38,20,12,8,16,10,20,0,4,4, %T A213270 14,0,10 %N A213270 Costas arrays such that the corresponding permutation is an involution. %C A213270 Self-inverse permutations such that each row in the difference table consists of pairwise distinct elements (see example). %H A213270 Scott Rickard, <a href="http://costasarrays.org/">costasarrays.org</a> (information and papers about Costas arrays). %e A213270 The permutation (4, 7, 9, 1, 6, 5, 2, 8, 3) is an involution and corresponds to a Costas array: %e A213270 4 7 9 1 6 5 2 8 3 (Permutation: p(1), p(2), p(3), ..., p(n) ) %e A213270 3 2 -8 5 -1 -3 6 -5 (step-1 differences: p(2)-p(1), p(3)-p(2), ... ) %e A213270 5 -6 -3 4 -4 3 1 (step-2 differences: p(3)-p(1), p(4)-p(2), ... ) %e A213270 -3 -1 -4 1 2 -2 (step-3 differences: p(4)-p(1), p(5)-p(2), ... ) %e A213270 2 -2 -7 7 -3 ( etc. ) %e A213270 1 -5 -1 2 %e A213270 -2 1 -6 %e A213270 4 -4 %e A213270 -1 %Y A213270 Cf. A008404 (Costas arrays), A213271 (Costas arrays that are derangements), A213338 (Costas arrays that are cyclic), A213339 (Costas arrays that are connected). %K A213270 nonn,hard,more %O A213270 1,2 %A A213270 _Joerg Arndt_, Jun 08 2012