Duplicate of A056182.

Terms

a(0) =2a(1) =10a(2) =38a(3) =130a(4) =422a(5) =1330a(6) =4118a(7) =12610a(8) =38342a(9) =116050a(10) =350198a(11) =1054690a(12) =3172262a(13) =9533170a(14) =28632278a(15) =85962370a(16) =258018182a(17) =774316690a(18) =2323474358a(19) =6971471650a(20) =20916512102a(21) =62753730610a(22) =188269580438a(23) =564825518530