Monday Math 89

A divisor function σx(n) is an arithmetic function consisting of the sum of the xth powers of the positive divisors of n:

Notable special cases include , known as the divisor function, which gives the number of positive divisors of n; and , the sum-of-divisors function, which adds all divisors of n, including n.
[The aliquot sum, the sum of all proper divisors of n, is thus s(n)=σ(n)-n; a “perfect” number is one for which s(n)=n.]
For example, consider n=12. The divisors are 1,2,3,4,6,12, and so:

One can in fact define this for any real, or even complex, x. Note that for real x>0, we can see by multiplying σx(n) by nx, that

If we have n=ab, gcd(a,b)=1, with a having divisors 1,a1,a2,…,a and b having divisors 1,b1,b2,…,b, then we see that every product aibj is a divisor of n, and that every divisor of n is such a product. Thus, we can show that divisor functions σx(n) are multiplicative functions.
For powers of primes, we see pk has k+1 factors: 1,p,p2,…,pk-1,pk. Thus

(for x>0).
Considering, then, the prime factorization , we get

[Note that , so that the x=0 case is the limit of the x≠0 general case].

Recall previouly that the Dirichlet series generating function for a sequence consisting of a multiplicative function can be represented as a product over primes:
For a divisor function, we see that
Now, we need to use the formula for the product of power series:
, where .
Letting ak=1, bk=bk for some positive b, then:
. (Note that both series on the left are geometric.)
Now, we see that since , the sum inside the product for our Dirichlet series is equivalent to the right-hand side of the above with and . Thus,
Taking the product over the primes, we thus see that
This means that



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2 Responses to “Monday Math 89”

  1. Monday Math 90 « Twisted One 151’s Weblog Says:

    […] Math 90 By twistedone151 Last week, I talked about the divisor functions σx(n), defined as the sum of the xth powers of the […]

  2. Monday Math 94 « Twisted One 151's Weblog Says:

    […] function of the divisor function σx(n) is becomes much more simple than the proof seen here: Since , we see ; since the Dirichlet series generating function of 1(n) is , and the Dirichlet […]

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