a(n) = floor(Li(2^n)), where Li(x) is the integral from 0 to x of dt/log(t).

Terms

a(0) =1a(1) =2a(2) =5a(3) =8a(4) =13a(5) =21a(6) =36a(7) =60a(8) =103a(9) =181a(10) =321a(11) =576a(12) =1047a(13) =1919a(14) =3544a(15) =6583a(16) =12296a(17) =23069a(18) =43453a(19) =82137a(20) =155739a(21) =296113a(22) =564411a(23) =1078221a(24) =2063984a(25) =3958349a(26) =7604383a(27) =14631777a(28) =28194305a(29) =54401475