Restricted permutations.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =44a(6) =80a(7) =144a(8) =260a(9) =476a(10) =872a(11) =1600a(12) =2940a(13) =5404a(14) =9936a(15) =18272a(16) =33604a(17) =61804a(18) =113672a(19) =209072a(20) =384540a(21) =707276a(22) =1300880a(23) =2392688a(24) =4400836a(25) =8094396a(26) =14887912a(27) =27383136a(28) =50365436a(29) =92636476