Number of up-down involutions of length n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =3a(6) =6a(7) =11a(8) =24a(9) =51a(10) =120a(11) =283a(12) =716a(13) =1833a(14) =4948a(15) =13561a(16) =38788a(17) =112745a(18) =339676a(19) =1039929a(20) =3283876a(21) =10532747a(22) =34717276a(23) =116158851a(24) =398257012a(25) =1385117947a(26) =4925094508a(27) =17752742867a(28) =65297807204a(29) =243319812785