4-13. Given the cumulative distribution function F(X) =

⎧⎪⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎪⎩

0, X < 0;

1 2, 0≤ X < 1;

3 4, 1≤ X < 4;

1, X ≥ 4, determine the associated probability mass function. (Hint: Find the points of discontinuity. At such points f (x) > 0.)