Number of involutions avoiding the pattern 4231.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =21a(6) =51a(7) =128a(8) =327a(9) =858a(10) =2272a(11) =6146a(12) =16716a(13) =46246a(14) =128414a(15) =361493a(16) =1020506a(17) =2913060a(18) =8335405a(19) =24067930a(20) =69646035