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 A003109 M5045 #29 May 17 2019 16:26:45 %S A003109 1,1,17,117,1413,46389,1211085,47977305,2302999889,137682614769, %T A003109 9844042388505,832087399629125 %N A003109 a(n) = number of special even permutations of 2*n+1. %C A003109 a(n) is the number of even permutations, pi(0),...,pi(2*n), of 0,...,2*n such that pi(0) = 0 and p(k) = k + pi(k) (mod 2*n+1), k=0,...,2*n is also a permutation of 0,...,2*n. - _Sean A. Irvine_, Jan 30 2015 %D A003109 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A003109 D. H. Lehmer, <a href="http://dx.doi.org/10.1016/0022-314X(73)90056-5">Some properties of circulants</a>, J. Number Theory 5 (1973), 43-54. %Y A003109 Cf. A003110 (special odd permutations), A003111 (total). %K A003109 nonn,more %O A003109 1,3 %A A003109 _N. J. A. Sloane_ %E A003109 a(8)-a(9) from _Sean A. Irvine_, Jan 31 2015 %E A003109 a(10)-a(12) from _Vaclav Kotesovec_, May 17 2019 (using the terms of A003111 and A003112)