Number of permutations of 1..n that almost avoid 231.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =111a(6) =531a(7) =2519a(8) =11726a(9) =53547a(10) =240448a(11) =1064608a(12) =4658952a(13) =20192022a(14) =86807865a(15) =370665585a(16) =1573606410a(17) =6647552115a(18) =27962334180a(19) =117185243340a(20) =489508952160