a(n) = Sum_{j=0..2*i, i=0..n} A026584(i,j).

Terms

a(0) =1a(1) =3a(2) =9a(3) =23a(4) =61a(5) =155a(6) =401a(7) =1023a(8) =2629a(9) =6723a(10) =17241a(11) =44135a(12) =113101a(13) =289643a(14) =742049a(15) =1900623a(16) =4868821a(17) =12471315a(18) =31946601a(19) =81831863a(20) =209618269a(21) =536945723a(22) =1375418801a(23) =3523201695a(24) =9024876901a(25) =23117683683a(26) =59217191289a(27) =151687926023