%I
%S 0,0,0,12,48,720,4320,60480,483840,7257600,72576000,1197504000,
%T 14370048000,261534873600,3661488230400,73229764608000,
%U 1171676233728000,25609494822912000,460970906812416000
%N Number of adjacent pairs of form (even,even) among all permutations of {1,2,...,n}.
%F a(n) = floor(n/2)*floor(n/21)*(n1)!. Proof: There are floor(n/2)*floor(n/21) pairs (r, s) with r and s even and distinct. For each pair, there are n1 places it can occur in a permutation and (n2)! possible arrangements of the other numbers.
%F a(n) = A110660(n+2) * A000142(n1).  _Michel Marcus_, Aug 29 2013
%o (PARI) a(n) = n\2 * (n\21)*(n1)! ; \\ _Michel Marcus_, Aug 29 2013
%Y Cf. A077611, A077613.
%K nonn
%O 1,4
%A _Leroy Quet_, _Frank Ruskey_ and _Dean Hickerson_, Nov 11 2002
