Number of involutions avoiding 3421.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =25a(6) =66a(7) =173a(8) =460a(9) =1218a(10) =3240a(11) =8602a(12) =22878a(13) =60794a(14) =161668a(15) =429752a(16) =1142758a(17) =3038173a(18) =8078606a(19) =21479469a(20) =57113888