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 A101609 #7 Feb 06 2017 20:14:00
%S A101609 0,2,6,36,180,1320,9240,84000,756000,8285760,91143360,1173553920,
%T A101609 15256200960,226040855040,3390612825600,56865153945600,
%U A101609 966707617075200,18112111963545600,344130127307366400,7125892746964992000
%N A101609 a(n) = n! * Sum_{k=1..floor(n/2)} 1/k.
%F A101609 a(1) = 0, a(n) = a(n-1)*n + (1 + (-1)^n)*(n-1)!. - _Daniel Suteu_, Feb 06 2017
%t A101609 Table[ n! HarmonicNumber[ Floor[ n/2]], {n, 20}] (* _Robert G. Wilson v_, Dec 11 2004 *)
%Y A101609 Cf. A101610, A101611, A101612, A101613, A000254, A046673, A052517.
%K A101609 nonn,easy
%O A101609 1,2
%A A101609 _Ralf Stephan_, Dec 10 2004