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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =0a(6) =-5a(7) =-20a(8) =-55a(9) =-125a(10) =-250a(11) =-450a(12) =-725a(13) =-1000a(14) =-1000a(15) =0a(16) =3625a(17) =13125a(18) =34375a(19) =76875a(20) =153750a(21) =278125a(22) =450000a(23) =621875a(24) =621875a(25) =0a(26) =-2250000a(27) =-8140625a(28) =-21312500a(29) =-47656250