Visible Lattice Points(spoj7001+初探莫比乌斯)gcd(a,b,c)=1 经典
VLATTICE-VisibleLatticePointsnotags ConsideraN*N*Nlattice.Onecornerisat(0,0,0)andtheoppositeoneisat(N,N,N).Howmanylatticepointsarevisiblefromcornerat(0,0,0)?ApointXisvisiblefrompointYiffnootherlattic