= Theta constant
{title2=$\Theta[\alpha,\beta](\tau)=\theta[\alpha,\beta](0,\tau)$}
A <theta function with characteristics> evaluated at spatial argument zero. Its dependence on $\tau\in\mathbb H$ remains <holomorphic>. Among the four half-integer characteristic choices, the odd one vanishes identically, whereas the three even constants are nonzero on the half-plane. The <zeros of theta functions with characteristics> locate their spatial zero away from the origin. Ratios of equal powers of even constants yield the <modular lambda function> and other <modular-invariant functions>.
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