a(n) = A128941(n) + 2.

Terms

a(0) =6a(1) =30a(2) =140a(3) =631a(4) =2786a(5) =12136a(6) =52368a(7) =224406a(8) =956516a(9) =4060038a(10) =17175132a(11) =72454075a(12) =304941386a(13) =1280898304a(14) =5371301504a(15) =22491017758a(16) =94055344244a(17) =392888085100