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 A173841 #16 Sep 10 2025 09:13:26
%S A173841 1,1,0,2,8,48,240,1968,13824,140160,1263360,15298560,168422400,
%T A173841 2373073920,30865121280,496199854080,7445355724800,134510244986880,
%U A173841 2287168006717440,45877376537395200,871804170613555200,19225435113632563200,403779880746418176000
%N A173841 Number of permutations of 1..n with no adjacent pair summing to n+1.
%C A173841 If a(n,k) is the number of permutations of 1..n with no adjacent pair summing to n+k, then a(n,k) = a(n,k+1) for n+k even. [proved by William Keith]
%F A173841 k = 1; a(n,k) = Sum_{j=0..m} (-2)^j*binomial(m,j)*(n-j)! where m = max(0, floor((n-k+1)/2)). [From _Max Alekseyev_, on the Sequence Fans Mailing List]
%F A173841 a(2n) = A007060(n); a(2n-1) = A285850(n)/(2n). - _David Radcliffe_, Sep 09 2025
%Y A173841 Cf. A007060 (bisection), A285850.
%K A173841 nonn,changed
%O A173841 0,4
%A A173841 _R. H. Hardin_, Feb 26 2010
%E A173841 More terms from _Alois P. Heinz_, Jan 09 2017