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Concrete mathematics : a foundation for computer science

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144 NUMBER THEORY<br />

But the inner sum is congruent to 0 modulo mz, because we’ve assumed that<br />

(4.64) holds <strong>for</strong> ml; so the entire sum is congruent to 0 modulo m2. By a<br />

symmetric argument, we find that the entire sum is congruent to 0 modulo ml<br />

as well. Thus by (4.42) it’s ‘congruent to 0 modulo m. QED.<br />

Exercises<br />

Warmups<br />

1 What is the smallest positive integer that has exactly k divisors, <strong>for</strong><br />

l

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