a(n) = 1 + 2^(2^n) + 2^(2^(n+1)).A220161a(n) = 1 + 2^(2^n) + 2^(2^(n+1)).Terms a(0) =7a(1) =21a(2) =273a(3) =65793a(4) =4295032833External referencesoeis: A220161