a(n) = Sum_{j=0..n} Sum_{k=0..j} A026615(j, k).

Terms

a(0) =1a(1) =3a(2) =8a(3) =20a(4) =46a(5) =100a(6) =210a(7) =432a(8) =878a(9) =1772a(10) =3562a(11) =7144a(12) =14310a(13) =28644a(14) =57314a(15) =114656a(16) =229342a(17) =458716a(18) =917466a(19) =1834968a(20) =3669974a(21) =7339988a(22) =14680018a(23) =29360080a(24) =58720206a(25) =117440460a(26) =234880970a(27) =469761992a(28) =939524038a(29) =1879048132