a(n) = 1 if n is the product of an even number of distinct primes, otherwise a(n) = -1.

Terms

a(0) =1a(1) =-1a(2) =-1a(3) =-1a(4) =-1a(5) =1a(6) =-1a(7) =-1a(8) =-1a(9) =1a(10) =-1a(11) =-1a(12) =-1a(13) =1a(14) =1a(15) =-1a(16) =-1a(17) =-1a(18) =-1a(19) =-1a(20) =1a(21) =1a(22) =-1a(23) =-1a(24) =-1a(25) =1a(26) =-1a(27) =-1a(28) =-1a(29) =-1