a(n) = Sum[2^(A001651(i-1)-1), {i,1,n}].

Terms

a(0) =1a(1) =3a(2) =11a(3) =27a(4) =91a(5) =219a(6) =731a(7) =1755a(8) =5851a(9) =14043a(10) =46811a(11) =112347a(12) =374491a(13) =898779a(14) =2995931a(15) =7190235a(16) =23967451a(17) =57521883a(18) =191739611a(19) =460175067a(20) =1533916891a(21) =3681400539a(22) =12271335131a(23) =29451204315a(24) =98170681051a(25) =235609634523