A317282 - OEIS

A317282

Number of permutations of [n] with exactly two increasing runs of even length.

5, 43, 258, 1853, 15634, 133697, 1207256, 11974457, 126980013, 1421291622, 16925790480, 213992804118, 2851968574855, 39996895523591, 589713707701692, 9114791365659163, 147335664946835342, 2486973983349027871, 43763134470537718258, 801419373432471123541

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OFFSET

4,1

LINKS

Alois P. Heinz, Table of n, a(n) for n = 4..472

MAPLE

b:= proc(u, o, t) option remember; series(`if`(u+o=0, x^t,

add(b(u+j-1, o-j, irem(t+1, 2)), j=1..o)+

add(b(u-j, o+j-1, 0)*x^t, j=1..u)), x, 3)

end:

a:= n-> coeff(b(n, 0$2), x, 2):

seq(a(n), n=4..30);