First differences of A000213, also twice A000073.

Terms

a(0) =0a(1) =0a(2) =2a(3) =2a(4) =4a(5) =8a(6) =14a(7) =26a(8) =48a(9) =88a(10) =162a(11) =298a(12) =548a(13) =1008a(14) =1854a(15) =3410a(16) =6272a(17) =11536a(18) =21218a(19) =39026a(20) =71780a(21) =132024a(22) =242830a(23) =446634a(24) =821488a(25) =1510952a(26) =2779074a(27) =5111514a(28) =9401540a(29) =17292128