Euler function

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Modulus of phi on the complex plane, colored so that black=0, red=4
For other meanings, see List of topics named after Leonhard Euler.

In mathematics, the Euler function is given by

\phi(q)=\prod_{k=1}^\infty (1-q^k).

Named after Leonhard Euler, it is a prototypical example of a q-series, a modular form, and provides the prototypical example of a relation between combinatorics and complex analysis.


The coefficient p(k) in the formal power series expansion for 1/\phi(q) gives the number of all partitions of k. That is,

\frac{1}{\phi(q)}=\sum_{k=0}^\infty p(k) q^k

where p(k) is the partition function of k.

The Euler identity, also known as the Pentagonal number theorem is

\phi(q)=\sum_{n=-\infty}^\infty (-1)^n q^{(3n^2-n)/2}.

Note that (3n^2-n)/2 is a pentagonal number.

The Euler function is related to the Dedekind eta function through a Ramanujan identity as

\phi(q)= q^{-\frac{1}{24}} \eta(\tau)

where q=e^{2\pi i\tau} is the square of the nome.

Note that both functions have the symmetry of the modular group.

The Euler function may be expressed as a Q-Pochhammer symbol:


The logarithm of the Euler function is the sum of the logarithms in the product expression, each of which may be expanded about q=0, yielding:


which is a Lambert series with coefficients -1/n. The logarithm of the Euler function may therefore be expressed as:

\ln(\phi(q))=\sum_{n=1}^\infty b_n q^n


b_n=-\sum_{d|n}\frac{1}{d}= -[1/1, 3/2, 4/3, 7/4, 6/5, 12/6, 8/7, 15/8, 13/9, 18/10, ...] (see OEIS A000203)

On account of the following identity,

\sum_{d|n} d = \sum_{d|n} \frac n d

this may also be written as

\ln(\phi(q))=-\sum_{n=1}^\infty \frac{q^n}{n} \sum_{d|n} d

Special values[edit]

The next identities come from Ramanujan's lost notebook, Part V, p. 326.