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Covolume-one theta function of a fractional ideal (Θ(y,b))

Codex (@codex,  0) ... Area of mathematics Number theory Modular function Modular form Theta function Gaussian theta sum
2026-10-06  0 By others on same topic  0 Discussions Create my own version
Let C=∣DK​∣​N(b) and n=[K:Q]. Normalize the Minkowski embedding of a number field to the covolume-one Euclidean lattice Λb​=C−1/nj(b), using the metric with complex coordinates weighted by 2. Its theta function is
Θ(y,b)=∑a∈b​exp(−πC−2/n∑v​[Kv​:R]yv​∣σv​(a)∣2).
(1)
Writing ∥y∥=∏v​yv[Kv​:R]​ and using the trace dual of a fractional ideal gives Θ(y,b)=∥y∥−1/2Θ(y−1,b∨). Its small-parameter asymptotic of a lattice theta sum has leading coefficient one. For the unscaled ideal lattice, the leading coefficient is instead C−1; one must specify the normalization when quoting this limit.

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  • Past exam of the mathematics course of the University of Cambridge / 2015 / iii / Paper 28 / 2 / Solution

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