a(0) = 0, a(1) = 1, a(2) = 2; for n > 2, a(n) = a(n-1) + 2*a(n-2) + a(n-3).

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =9a(5) =19a(6) =41a(7) =88a(8) =189a(9) =406a(10) =872a(11) =1873a(12) =4023a(13) =8641a(14) =18560a(15) =39865a(16) =85626a(17) =183916a(18) =395033a(19) =848491a(20) =1822473a(21) =3914488a(22) =8407925a(23) =18059374a(24) =38789712a(25) =83316385a(26) =178955183a(27) =384377665a(28) =825604416a(29) =1773314929