Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2016/iii/paper-124/5/d/solution

Let , , and let be the cumulative distribution function of . Since the prime omega function is integer-valued, is constant on every half-open interval between consecutive points of the lattice .
Choose and put , . Then , and has the same value throughout . The standard normal distribution function is continuous, so the definition of the Kolmogorov distance gives
where the second inequality follows by taking a limit from below at . The triangle inequality therefore yields
Thus the integer lattice forces the lower bound:
This lattice obstruction to normal approximation does not require a quantitative Erdős-Kac theorem; it follows directly from discreteness and the positive standard normal density near zero.

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