a(n) = 2^(n-4)*(4*n^2 - 16*n + 23).

Terms

a(0) =23a(1) =86a(2) =284a(3) =856a(4) =2416a(5) =6496a(6) =16832a(7) =42368a(8) =104192a(9) =251392a(10) =596992a(11) =1398784a(12) =3239936a(13) =7430144a(14) =16891904a(15) =38109184a(16) =85393408a(17) =190185472a(18) =421265408a(19) =928514048a(20) =2037383168a(21) =4452253696a(22) =9693036544a(23) =21030240256a(24) =45483032576a(25) =98079604736a(26) =210923159552