Theta function of a lattice

One can associate to any (positive-definite) lattice Λ a theta function given by

${\displaystyle \Theta _{\Lambda }(\tau )=\sum _{x\in \Lambda }e^{i\pi \tau \|x\|^{2}}\qquad \mathrm {Im} \,\tau >0.}$

The theta function of a lattice is then a holomorphic function on the upper half-plane. Furthermore, the theta function of an even unimodular lattice of rank n is actually a modular form of weight n/2. The theta function of an integral lattice is often written as a power series in ${\displaystyle q=e^{2i\pi \tau }}$ so that the coefficient of qⁿ gives the number of lattice vectors of norm 2n.

• Deconinck, Bernard (2010), "Multidimensional Theta Functions", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248.