a(n) = A081881(n) - 1.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =25a(5) =68a(6) =185a(7) =503a(8) =1368a(9) =3719a(10) =10110a(11) =27482a(12) =74704a(13) =203067a(14) =551994a(15) =1500476a(16) =4078717a(17) =11087103a(18) =30137871a(19) =81923227a(20) =222690420a(21) =605335322a(22) =1645472006a(23) =4472856654a(24) =12158484964a(25) =33050188740a(26) =89839727479a(27) =244209698680a(28) =663830786256