Multiple zeta function

For a different but related multiple zeta function, see Barnes zeta function.

In mathematics, the multiple zeta functions generalisations of the Riemann zeta function, defined by

\zeta(s_1, \ldots, s_k) = \sum_{n_1 > n_2 > \cdots > n_k > 0} \ \frac{1}{n_1^{s_1} \cdots n_k^{s_k}} = \sum_{n_1 > n_2 > \cdots > n_k > 0} \ \prod_{i=1}^k \frac{1}{n_i^{s_i}}, \!

and converge when Re(s1)+...+Re(si) > i for all i. Like the Riemann zeta function, the multiple zeta functions can be analytically continued to be meromorphic functions (see, for example, Zhao (1999)). When s1, ..., si are all positive integers these sums are often called multiple zeta values (MZVs) or Euler sums.

The standard shorthand for writing multiple zeta functions is to place repeating strings of the argument within braces and use a superscript to indicate the number of repetitions. For example,

\zeta(2,1,2,1,3) = \zeta(\{2,1\}^2,3)

and

\zeta(2,1,1,3,1,1) = \zeta(2,\{1\}^2,3,\{1\}^2).

Multiple zeta functions are known to satisfy what is known as MZV duality, the simplest case of which is the famous identity of Euler:

\sum_{n=1}^{\infty} \frac{H_n}{(n%2B1)^2} = \zeta(2,1) = \zeta(3) = \sum_{n=1}^{\infty} \frac{1}{n^3}, \!

where Hn are the harmonic numbers.

Special values of double zeta functions:

s1 s2 approximate value
2 2 0.811742425283353643637002772406
2 3 0.228810397603353759768746148942
2 4 0.088483382454368714294327839086
2 5 0.038575124342753255505925464373
3 2 0.711566197550572432096973806086
3 3 0.213798868224592547099583574508
3 4 0.085159822534833651406806018872
3 5 0.037707672984847544011304782294
4 2 0.674523914033968140491560608257
4 3 0.207505014615732095907807605495
4 4 0.083673113016495361614890436542

Mordell-Tornheim zeta values

The Mordell-Tornheim zeta function, introduced by Matsumoto (2003) who was motivated by the papers Mordell (1958) and Tornheim (1950), is defined by

\zeta_{MT,r}(s_1,\dots,s_r;s_{r%2B1})=\sum_{m_1,\dots,m_r>0}\frac{1}{ m_1^{s_1}\cdots m_r^{s_r}(m_1%2B\dots%2Bm_r)^{s_{r%2B1}}}

It is a special case of the Shintani zeta function.

References

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