a(n) = sum of absolute values of coefficients of (1+x-x^2)^n.

Terms

a(0) =1a(1) =3a(2) =7a(3) =13a(4) =35a(5) =83a(6) =165a(7) =367a(8) =899a(9) =1957a(10) =3839a(11) =9771a(12) =22709a(13) =43213a(14) =102963a(15) =255061a(16) =525601a(17) =1098339a(18) =2798273a(19) =6202969a(20) =11746259a(21) =29976073a(22) =70898649a(23) =140495779a(24) =314391789a(25) =787757461a(26) =1688887719a(27) =3337986541a(28) =8583687613a(29) =19647782463