a(n) = n*2^(n+1) + (2^(n+3)+(-1)^n)/3.

Terms

a(0) =3a(1) =9a(2) =27a(3) =69a(4) =171a(5) =405a(6) =939a(7) =2133a(8) =4779a(9) =10581a(10) =23211a(11) =50517a(12) =109227a(13) =234837a(14) =502443a(15) =1070421a(16) =2271915a(17) =4805973a(18) =10136235a(19) =21321045a(20) =44739243a(21) =93672789a(22) =195734187a(23) =408245589a(24) =850045611a(25) =1767200085a(26) =3668617899a(27) =7605671253a(28) =15748213419a(29) =32570168661