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

Terms

a(0) =1a(1) =4a(2) =10a(3) =26a(4) =80a(5) =194a(6) =504a(7) =1442a(8) =3710a(9) =9536a(10) =26842a(11) =69014a(12) =178704a(13) =496602a(14) =1316204a(15) =3377206a(16) =9242898a(17) =24629944a(18) =63304540a(19) =172497622a(20) =462822414a(21) =1210912388a(22) =3177522724a(23) =8736822276a(24) =22617998204a(25) =59776061150a(26) =163702751968a(27) =433787373560