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

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =116a(5) =344a(6) =1104a(7) =3280a(8) =10456a(9) =31152a(10) =98804a(11) =295988a(12) =935876a(13) =2811540a(14) =8870324a(15) =26695724a(16) =84060148a(17) =253376840a(18) =796635360a(19) =2404558304a(20) =7549431884a(21) =22820942416a(22) =71541295984a(23) =216562743948a(24) =677938097756