Number of involutions avoiding the pattern 2341.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =25a(5) =66a(6) =170a(7) =441a(8) =1124a(9) =2870a(10) =7273a(11) =18477a(12) =46825a(13) =118917a(14) =301734a(15) =766525a(16) =1946293a(17) =4944614a(18) =12557685