a(n) = a(n-1) + 20*a(n-2), n >= 2; a(0)=1, a(1)=1.

Terms

a(0) =1a(1) =1a(2) =21a(3) =41a(4) =461a(5) =1281a(6) =10501a(7) =36121a(8) =246141a(9) =968561a(10) =5891381a(11) =25262601a(12) =143090221a(13) =648342241a(14) =3510146661a(15) =16476991481a(16) =86679924701a(17) =416219754321