G.f.: (1+3*x+12*x^2+48*x^3)/(1-x-256*x^4).

Terms

a(0) =1a(1) =4a(2) =16a(3) =64a(4) =320a(5) =1344a(6) =5440a(7) =21824a(8) =103744a(9) =447808a(10) =1840448a(11) =7427392a(12) =33985856a(13) =148624704a(14) =619779392a(15) =2521191744a(16) =11221570880a(17) =49269495104a(18) =207933019456a(19) =853358105920