a(n) = (A174216(n)-1)/2.

Terms

a(0) =7a(1) =13a(2) =31a(3) =61a(4) =139a(5) =283a(6) =571a(7) =1153a(8) =2311a(9) =4723a(10) =9463a(11) =19141a(12) =38569a(13) =77419a(14) =154873a(15) =310231a(16) =620911a(17) =1241923a(18) =2483869a(19) =4967803a(20) =9946273a(21) =19892599