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 A275801 #29 Aug 13 2016 10:21:01
%S A275801 1,0,1,4,53,936,25325,933980,45504649,2824517520,217690037497,
%T A275801 20394614883316,2282650939846781,300814135522967736,
%U A275801 46103574973075123877,8130996533576437261772,1635028654501420083152785,371853339350614571322913824,94969025880924845123887493233
%N A275801 Number of alternating permutations of the multiset {1,1,2,2,...,n,n}.
%C A275801 Number of permutations (p(1),...,p(2n)) of {1,1,2,2,...,n,n} satisfying p(1) < p(2) > p(3) < ... < p(2n).
%C A275801 a(n) <= A005799(n) <= A275829(n).
%H A275801 Max Alekseyev, <a href="/A275801/b275801.txt">Table of n, a(n) for n = 0..70</a>
%H A275801 Wikipedia, <a href="http://en.wikipedia.org/wiki/Alternating_permutation">Alternating permutation</a>.
%H A275801 hkju et al., <a href="http://mathoverflow.net/q/247107">Number of updown sequences of 1,1,2,2,...,n,n</a>, Mathoverflow, 2016.
%Y A275801 Column k=2 of A275784.
%Y A275801 Cf. A000111, A001250, A000459, A004075, A005799, A114938, A137729, A137730, A137737, A137749, A275829.
%K A275801 nonn
%O A275801 0,4
%A A275801 _Max Alekseyev_, Aug 09 2016