Weierstrass sigma function (${\sigma }_{\Lambda }(z)=z{\prod }_{\omega \in \Lambda \setminus \{0\}}(1-z/\omega )e^{z/\omega +z^2/(2{\omega }^2)}$)

For a period lattice, the displayed canonical product converges normally because its logarithmic tails are bounded by a constant times $|\omega {|}^{-3}$. It defines an odd entire function with simple zeros exactly at the lattice and derivative one at zero. Its logarithmic derivative is the Weierstrass zeta function, and the derivative of that logarithmic derivative is minus the Weierstrass elliptic function. Sigma is quasi-periodic rather than elliptic; balanced products of its translates construct meromorphic functions with prescribed divisors on a complex torus.

For an oriented lattice basis with $\mathrm{Im}({\omega }_2/{\omega }_1)>0$, write ${\eta }_j=\zeta (z+{\omega }_j)-\zeta (z)$. These differences are constant because ${\zeta }^{\prime }=-\wp$ is periodic. Integrating zeta around a fundamental cell gives the displayed relation by its one simple pole of residue one. Oddness and the logarithmic derivative then give $\sigma (z+{\omega }_j)=-e^{{\eta }_j(z+{\omega }_j/2)}\sigma (z)$. This fixes both the sign and the exponent convention in sigma-product constructions.