a(n) = sum of absolute values of coefficients of (1+x-3x^2)^n.

Terms

a(0) =1a(1) =5a(2) =23a(3) =99a(4) =401a(5) =1525a(6) =6345a(7) =27331a(8) =122083a(9) =520805a(10) =2117293a(11) =8301441a(12) =34517395a(13) =147850771a(14) =628707981a(15) =2675100397a(16) =10920387779a(17) =43701876543a(18) =180872758207a(19) =769658883325