Mertens's function: Sum_{k=1..n} mu(k), where mu is the Moebius function A008683.

Terms

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