Geodesic
Behavior of theta series of degree~$n$ under modular substitutions
Izvestiya. Mathematics , Tome 9 (1975) no. 2, pp. 227-241.

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Let $F$ be an integral, symmetric, positive definite matrix of order $m\geqslant1$ with an even diagonal. For the theta series of $F$ of degree $n\geqslant1$ $$ \theta_F^{(n)}(Z)=\sum_x^F\exp(\pi i\operatorname{Tr}(^tXFXZ)), $$ where $X$ runs through all integral $m\times n$ matrices and $Z$ is a point of the Siegel upper halfplane of degree $n$, the congruence subgroup of the group $Sp_n(\mathbf Z)$ is found, with respect to which $\theta_F^{(n)}(Z)$ is a Siegel modular form with a multiplicator system (the analog of the group $\Gamma_0(q)$)). The analogous problem is solved for theta series of degree $n$ with spherical functions. The appropriate multiplicator systems are computed for even $m$. Bibliography: 5 items. 
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A. N. Andrianov; G. N. Maloletkin. Behavior of theta series of degree~$n$ under modular substitutions. Izvestiya. Mathematics , Tome 9 (1975) no. 2, pp. 227-241. http://geodesic.mathdoc.fr/item/IM2_1975_9_2_a0/

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