Number of ternary strings of length n that contain at least one 0 and at most two 1's.

Terms

a(0) =0a(1) =1a(2) =5a(3) =19a(4) =61a(5) =176a(6) =474a(7) =1219a(8) =3035a(9) =7378a(10) =17608a(11) =41405a(12) =96177a(13) =221092a(14) =503702a(15) =1138567a(16) =2555767a(17) =5701478a(18) =12648276a(19) =27918145a(20) =61341485a(21) =134217496a(22) =292552450a(23) =635436779a(24) =1375731411a(25) =2969566906a(26) =6392118944a(27) =13723762309a(28) =29393682025a(29) =62813896268