Sum of Floor[ 3*(1+sqrt(2))^(n-k) ] for k from 1 to infinity.

Terms

a(0) =4a(1) =11a(2) =28a(3) =70a(4) =171a(5) =417a(6) =1010a(7) =2444a(8) =5905a(9) =14263a(10) =34440a(11) =83154a(12) =200759a(13) =484685a(14) =1170142a(15) =2824984a(16) =6820125a(17) =16465251a(18) =39750644a(19) =95966558a(20) =231683779a(21) =559334137a(22) =1350352074a(23) =3260038308a(24) =7870428713a(25) =19000895759