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Tuesday, 3 October 2006

nt.number theory - Transformation formulae for classical theta functions

I am looking for a reference for the transformation formulae
for the classical theta-functions
theta4(tau)=sumin=inftynfty(1)nqn2
and
theta2(tau)=sumin=inftynftyq(2n+1)2/4
under the congruence group Gamma0(4).
Here tau lies in the upper-half plane and qx denotes
exp(2piixtau). More precisely I want the exact automorphy
factors for each AinGamma0(4) (some eighth root of
unity times sqrtctau+d). I know these can easily
be deduced from those for the basic theta-function
theta3(tau)=sumin=inftynftyqn2
for which a nice reference for the automorphy factors is Koblitz's Introduction
to Elliptic Curves and Modular Forms
. However



  1. a citation would be useful to me,


  2. I want to check my calculation and


  3. a reference may give the formulae in a more convenient form than I have.


Thanks in advance.



EDIT I have now found a convenient reference: Rademacher's
Topics in Analytic Number Theory.



FURTHER EDIT Rademacher atcually gives full transformation formula
for the two-variable classical Jacobi theta functions under arbitrary
matrices in mathrmSL2(mathbbZ). From these we can deduce
for AinGamma1(4) that
fractheta2(Atau)theta3(Atau)=ibfractheta2(tau)theta3(tau)
and
fractheta4(Atau)theta3(Atau)=ic/4fractheta4(tau)theta3(tau)
in the usual notation. Once noticed, these relations are easy to prove
from scratch.



Thanks to all who replied to this question.

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