Restricted permutations.

Terms

a(0) =6a(1) =11a(2) =20a(3) =36a(4) =65a(5) =119a(6) =218a(7) =400a(8) =735a(9) =1351a(10) =2484a(11) =4568a(12) =8401a(13) =15451a(14) =28418a(15) =52268a(16) =96135a(17) =176819a(18) =325220a(19) =598172a(20) =1100209a(21) =2023599a(22) =3721978a(23) =6845784a(24) =12591359a(25) =23159119a(26) =42596260a(27) =78346736a(28) =144102113a(29) =265045107