Convolution sequence, A000027 / A008683.

Terms

a(0) =1a(1) =3a(2) =7a(3) =14a(4) =27a(5) =49a(6) =88a(7) =155a(8) =272a(9) =472a(10) =819a(11) =1415a(12) =2446a(13) =4219a(14) =7280a(15) =12548a(16) =21635a(17) =37283a(18) =64264a(19) =110742a(20) =190860a(21) =328893a(22) =566800a(23) =976727a(24) =1683204a(25) =2900567a(26) =4998510a(27) =8613679a(28) =14843737a(29) =25579540