(Smallest prime >= 2^n) + (largest prime <= 2^n).

Terms

a(0) =2a(1) =4a(2) =8a(3) =18a(4) =30a(5) =68a(6) =128a(7) =258a(8) =508a(9) =1030a(10) =2052a(11) =4092a(12) =8192a(13) =16400a(14) =32792a(15) =65520a(16) =131058a(17) =262172a(18) =524286a(19) =1048596a(20) =2097156a(21) =4194312a(22) =8388620a(23) =16777210a(24) =33554472a(25) =67108860a(26) =134217738a(27) =268435446a(28) =536870858a(29) =1073741832