Consider the following mathematical model minimize x1 + 3x2 + f(x3) + g(x4) subject to the following constraints (a) Either x1 + 2x2 + 3x3 >= 6 or 3x2 + 4x3 >= 9. (b) At least two of the following four inequalities hold. 5x1 + 6x3 + x4 >= 9, 3x1 + x2 + 4x3 <= 10, x2 + x3 >= 1, 3x3 >= 5. (c) If x2 != 2, then x1 >= 3. (d) x4 = 0 or 1 or 2. (e) x1, x2 and x3 are integers. (f) xi >= 0, i = 1, 2, 3, 4. where f(x3) = { 8 + 3x3 if x3 > 0; 3 otherwise, and g(x4) = x4^2. Formulate this as an integer programming problem.