a(n) = ((6*n+1)*4^n - 1)/3.

Terms

a(0) =0a(1) =9a(2) =69a(3) =405a(4) =2133a(5) =10581a(6) =50517a(7) =234837a(8) =1070421a(9) =4805973a(10) =21321045a(11) =93672789a(12) =408245589a(13) =1767200085a(14) =7605671253a(15) =32570168661a(16) =138870609237a(17) =589842175317