Number of involutions avoiding the pattern 1342.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =24a(6) =62a(7) =156a(8) =406a(9) =1040a(10) =2714a(11) =7012a(12) =18322a(13) =47560a(14) =124358a(15) =323708a(16) =846766a(17) =2208032a(18) =5777330a(19) =15082372a(20) =39469786