a(n) = Sum_{k >= 0}(-1)^k*binomial(n,5*k+1).

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =4a(5) =5a(6) =5a(7) =0a(8) =-20a(9) =-75a(10) =-200a(11) =-450a(12) =-900a(13) =-1625a(14) =-2625a(15) =-3625a(16) =-3625a(17) =0a(18) =13125a(19) =47500a(20) =124375a(21) =278125a(22) =556250a(23) =1006250a(24) =1628125a(25) =2250000a(26) =2250000a(27) =0a(28) =-8140625a(29) =-29453125