Duplicate of A006443.

Terms

a(0) =2a(1) =4a(2) =14a(3) =47a(4) =184a(5) =761a(6) =3314a(7) =14997a(8) =69886a(9) =333884a(10) =1626998a(11) =8067786a(12) =40580084a(13) =206734083a(14) =1064666724a(15) =5536480877a(16) =29036188788