Second column of A218869.

Terms

a(0) =0a(1) =2a(2) =4a(3) =12a(4) =20a(5) =52a(6) =100a(7) =214a(8) =414a(9) =876a(10) =1722a(11) =3524a(12) =6992a(13) =14144a(14) =28188a(15) =56720a(16) =113180a(17) =227084a(18) =453748a(19) =908804a(20) =1816726a(21) =3636336a(22) =7270770a(23) =14547228a(24) =29090912a(25) =58192994a(26) =116378984a(27) =232780848a(28) =465546796a(29) =931139720