a(n) = 4*a(n-1) + 2*a(n-2) for n > 1; a(0) = 1, a(1) = 6.

Terms

a(0) =1a(1) =6a(2) =26a(3) =116a(4) =516a(5) =2296a(6) =10216a(7) =45456a(8) =202256a(9) =899936a(10) =4004256a(11) =17816896a(12) =79276096a(13) =352738176a(14) =1569504896a(15) =6983495936a(16) =31072993536a(17) =138258966016a(18) =615181851136