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Thread: Primes help

  1. May 12th 2009, 04:32 PM #1
    amma0913
    amma0913 is offline
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    Primes help

    I need help with this question please!

    Create a formula (using sigma, logs, floor function) that computes the exponent of any particular prime in the prime factorization of a given factorial.

    we use the notation
    exp(p, n!) = the exponent of prime p in the prime factorization of n!

    Example: exp(5, 25!) = 6

    help please!
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  2. May 12th 2009, 08:01 PM #2
    Media_Man
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    A Simple Formula

    exp(p, n!) = \sum_{i=1}^{\infty} \lfloor \frac{n}{p^i}\rfloor

    For example, 625! contains 125 multiples of 5, 25 multiples of 25, 5 multiples of 125, and 1 multiple of 625, thus exp(5, 625!) = 125+25+5+1 = 156 .

    If you're actually going to use this in practice, the \infty is not necessary. Since \lfloor \frac{n}{p^i}\rfloor = 0 for n<p^i , you only have to sum up to i=\lfloor \frac{ln(n)}{ln(p)} \rfloor , but that doesn't make for as pretty of a formula.
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