Shifts 3 places left under exponentiation.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =5a(6) =15a(7) =53a(8) =213a(9) =961a(10) =4808a(11) =26405a(12) =157965a(13) =1022573a(14) =7122441a(15) =53118601a(16) =422362118a(17) =3566967917a(18) =31887812715a(19) =300848966213