Matchings avoiding the pattern 231.

Terms

a(0) =1a(1) =1a(2) =3a(3) =14a(4) =83a(5) =570a(6) =4318a(7) =35068a(8) =299907a(9) =2668994a(10) =24513578a(11) =230981316a(12) =2222973742a(13) =21777680644a(14) =216603095388