Partial sums of A005256.

Terms

a(0) =1a(1) =4a(2) =10a(3) =22a(4) =45a(5) =90a(6) =177a(7) =348a(8) =684a(9) =1350a(10) =2670a(11) =5298a(12) =10531a(13) =20974a(14) =41815a(15) =83452a(16) =166639a(17) =332926a(18) =665329a(19) =1329964a(20) =2658898a(21) =5316430a(22) =10630828a(23) =21258958a(24) =42513898a(25) =85022458a(26) =170036950a(27) =340063306a(28) =680110785a(29) =1360200510