a(n) = 49*2^n - 3*n - 18.

Terms

a(0) =31a(1) =77a(2) =172a(3) =365a(4) =754a(5) =1535a(6) =3100a(7) =6233a(8) =12502a(9) =25043a(10) =50128a(11) =100301a(12) =200650a(13) =401351a(14) =802756a(15) =1605569a(16) =3211198a(17) =6422459a(18) =12844984a(19) =25690037a(20) =51380146a(21) =102760367a(22) =205520812a(23) =411041705a(24) =822083494a(25) =1644167075a(26) =3288334240a(27) =6576668573a(28) =13153337242a(29) =26306674583