Binomial transform of double factorial n!! (A006882).

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =37a(5) =111a(6) =355a(7) =1191a(8) =4201a(9) =15445a(10) =59171a(11) =234983a(12) =966397a(13) =4101709a(14) =17946783a(15) =80754331a(16) =373286481a(17) =1769440513a(18) =8592681907a(19) =42689422871a(20) =216789872741