For x E R, set f (ac ) = (0 if 1/x EN, 1 if x = 0 or 1/x & N. So, f(ac) = 0 if x E {1, 1/2, 1/3, ...}, and f(a) = 1 for all other a E R. Show, only using the definition of continuity, that f is continuous at 2/3, but discontinuous at 0