Number of ballot sequences of length n with exactly 4 fixed points.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =1a(6) =3a(7) =9a(8) =29a(9) =98a(10) =350a(11) =1302a(12) =5062a(13) =20380a(14) =85108a(15) =366444a(16) =1627836a(17) =7430360a(18) =34855016a(19) =167546408a(20) =825185448a(21) =4155400720a(22) =21388745008a(23) =112355110672a(24) =602103194448