a(n) = sum of the first n coefficients of (1+x+x^2)^n.

Terms

a(0) =1a(1) =3a(2) =10a(3) =31a(4) =96a(5) =294a(6) =897a(7) =2727a(8) =8272a(9) =25048a(10) =75747a(11) =228826a(12) =690691a(13) =2083371a(14) =6280650a(15) =18925047a(16) =57002616a(17) =171633840a(18) =516632307a(19) =1554702516a(20) =4677501237a(21) =14069962041a(22) =42314975352a(23) =127240600050a(24) =382555886571