A275829 Number of weakly alternating permutations of the multiset {1,1,2,2,...,n,n}.

1, 1, 2, 12, 140, 2564, 68728, 2539632, 123686800, 7677924688, 591741636128, 55438330474944, 6204888219697856, 817697605612952384, 125322509904814743424, 22102340129003429880576, 4444468680409243484516608, 1010802175212828388101900544, 258152577318424951261637001728

Offset: 0

Author

Max Alekseyev, Aug 11 2016

Keywords

• nonn

Comments

Number of permutations (p(1),...,p(2n)) of {1,1,2,2,...,n,n} satisfying p(1) <= p(2) >= p(3) <= p(4) >= p(5) <= ... <= p(2n).

a(n) >= A005799(n) >= A275801(n).

Links

• Max Alekseyev, Table of n, a(n) for n = 0..70

Crossrefs

Cf. A000111, A001250, A000459, A004075, A005799, A114938, A137729, A137730, A137737, A137749, A275801.