Number of involutions avoiding the pattern 2431.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =24a(5) =62a(6) =154a(7) =396a(8) =992a(9) =2536a(10) =6376a(11) =16238a(12) =40914a(13) =103954a(14) =262298a(15) =665478a(16) =1680726a(17) =4260262a(18) =10766470a(19) =27274444