Number of involutions avoiding the pattern 1324.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =21a(6) =51a(7) =126a(8) =321a(9) =820a(10) =2160a(11) =5654a(12) =15272a(13) =40758a(14) =112280a(15) =304471a(16) =852164a(17) =2341980a(18) =6640755a(19) =18460066a(20) =52915999