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 A346720 #9 Sep 16 2021 03:47:21 %S A346720 0,1,2,163,6724,692741,86756038,16135231015,3838836369800, %T A346720 1178853270354697,447322130002332298,206783054242756958891, %U A346720 114117348385587556703692,74183362210489714472590093,56080450787901009525514063694,48790757690364513377740990959151,48400123863374755985614486072403728 %N A346720 a(n) is the number of negative Euler permutations of order 2*n. Bisection (even indices) of A347602. %C A346720 For definitions and comments see A347602. %F A346720 A346719(n) + a(n) = A000166(2n) (rencontres numbers). %F A346720 A346719(n) - a(n) = A000364(n) (Euler secant numbers). %F A346720 a(n) = subfactorial(2*n) / 2 - Im(PolyLog(-2*n, i)). %p A346720 # uses A000166. %p A346720 A346720 := n -> (A000166(2*n) - euler(2*n)) / 2: %p A346720 seq(A346720(n), n = 0..16); %t A346720 A346720[n_] := Subfactorial[2 n]/2 - Im[PolyLog[-2 n, I]]; %t A346720 Table[A346720[n], {n, 0, 16}] %Y A346720 Cf. A347601, A346719, A000166, A000364, A347602. %K A346720 nonn %O A346720 0,3 %A A346720 _Peter Luschny_, Sep 09 2021