a(0)=0 and a(n+1) = 3*a(n) + 2^(n+2).

Terms

a(0) =0a(1) =4a(2) =20a(3) =76a(4) =260a(5) =844a(6) =2660a(7) =8236a(8) =25220a(9) =76684a(10) =232100a(11) =700396a(12) =2109380a(13) =6344524a(14) =19066340a(15) =57264556a(16) =171924740a(17) =516036364a(18) =1548633380a(19) =4646948716a(20) =13942943300a(21) =41833024204a(22) =125507461220a(23) =376539160876