<h2>Number of divisors</h2>

The Tau function indicates the number of divisors a natural number n has. The divisors 1 and n, factors for any integer, are also taken into account, so that prime numbers will have exactly two factors. The function is denoted by the Greek letter &tau;.<br />
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It is defined in the following way:<br />
<img src="tau" />
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It holds that &tau; (p) = 2 for all primes.
Relation to the &phi;-function: &tau; (n) + &phi; (n) = n + 1 for all n in N.
