Number of ballot sequences of length 2n with exactly n fixed points.

A238803

Number of ballot sequences of length 2n with exactly n fixed points.

Terms

    a(0) =1a(1) =1a(2) =3a(3) =9a(4) =29a(5) =99a(6) =357a(7) =1351a(8) =5343a(9) =21993a(10) =93923a(11) =414969a(12) =1892277a(13) =8887291a(14) =42912261a(15) =212676951a(16) =1080355463a(17) =5617772049a(18) =29868493827a(19) =162204146857a(20) =898874710797

External references