a(n) = 4^(n+1)-2^n-1.

Terms

a(0) =2a(1) =13a(2) =59a(3) =247a(4) =1007a(5) =4063a(6) =16319a(7) =65407a(8) =261887a(9) =1048063a(10) =4193279a(11) =16775167a(12) =67104767a(13) =268427263a(14) =1073725439a(15) =4294934527a(16) =17179803647a(17) =68719345663a(18) =274877644799