In: Advanced Math
Let a, b, c ∈ Q, (a, b) /= (0, 0), and consider the line L = {(x, y) ∈ R2 ; ax + by = c}. State and prove a criterion in terms of the data a, b, c ∈ Q to check whether L∩Z2 = ∅, i.e., the line passes through no point with integer coordinates in the plane. Give an explicit example of such a line that is neither parallel to the x-axis nor to the y-axis!