a(n) is the sum of the first 2*n nonzero n-bonacci numbers.

Terms

a(0) =0a(1) =2a(2) =7a(3) =28a(4) =116a(5) =480a(6) =1968a(7) =8000a(8) =32320a(9) =130048a(10) =521984a(11) =2092032a(12) =8377344a(13) =33529856a(14) =134164480a(15) =536756224a(16) =2147237888a(17) =8589410304a(18) =34358624256a(19) =137436594176a(20) =549750833152