Partial sums of A001580.

Terms

a(0) =1a(1) =4a(2) =12a(3) =29a(4) =61a(5) =118a(6) =218a(7) =395a(8) =715a(9) =1308a(10) =2432a(11) =4601a(12) =8841a(13) =17202a(14) =33782a(15) =66775a(16) =132567a(17) =263928a(18) =526396a(19) =1051045a(20) =2100021a(21) =4197614a(22) =8392402a(23) =16781539a(24) =33559331a(25) =67114388a(26) =134223928a(27) =268442385a(28) =536878625a(29) =1073750378