a(n) = (1/2)*s(n+3), where s = A025248.

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =13a(5) =29a(6) =71a(7) =163a(8) =377a(9) =913a(10) =2219a(11) =5375a(12) =13189a(13) =32677a(14) =81167a(15) =202523a(16) =508273a(17) =1280537a(18) =3236275a(19) =8207543a(20) =20880893a(21) =53263405a(22) =136205527a(23) =349137811a(24) =896881641a(25) =2308523809a(26) =5953138875a(27) =15378562415a(28) =39791453685a(29) =103115768885