Let
K be a number field. For any system of semisimple mod
\ell Galois representations
\{{\it\phi}_{\ell }:\text{Gal}(\bar{\mathbb{Q}}/K)\rightarrow \text{GL}_{N}(\mathbb{F}_{\ell })\}_{\ell } arising from étale cohomology (Definition 1), there exists a finite normal extension
L of
K such that if we denote
{\it\phi}_{\ell }(\text{Gal}(\bar{\mathbb{Q}}/K)) and
{\it\phi}_{\ell }(\text{Gal}(\bar{\mathbb{Q}}/L)) by
\bar{{\rm\Gamma}}_{\ell } and
\bar{{\it\gamma}}_{\ell }, respectively, for all
\ell and let
\bar{\mathbf{S}}_{\ell } be the
\mathbb{F}_{\ell }-semisimple subgroup of
\text{GL}_{N,\mathbb{F}_{\ell }} associated to
\bar{{\it\gamma}}_{\ell } (or
\bar{{\rm\Gamma}}_{\ell }) by Nori’s theory [On subgroups of
\text{GL}_{n}(\mathbb{F}_{p}), Invent. Math. 88 (1987), 257–275] for sufficiently large
\ell, then the following statements hold for all sufficiently large
\ell.
A(i) The formal character of
\bar{\mathbf{S}}_{\ell }{\hookrightarrow}\text{GL}_{N,\mathbb{F}_{\ell }} (Definition 1) is independent of
\ell and equal to the formal character of
(\mathbf{G}_{\ell }^{\circ })^{\text{der}}{\hookrightarrow}\text{GL}_{N,\mathbb{Q}_{\ell }}, where
(\mathbf{G}_{\ell }^{\circ })^{\text{der}} is the derived group of the identity component of
\mathbf{G}_{\ell }, the monodromy group of the corresponding semi-simplified
\ell-adic Galois representation
{\rm\Phi}_{\ell }^{\text{ss}}.
A(ii) The non-cyclic composition factors of
\bar{{\it\gamma}}_{\ell } and
\bar{\mathbf{S}}_{\ell }(\mathbb{F}_{\ell }) are identical. Therefore, the composition factors of
\bar{{\it\gamma}}_{\ell } are finite simple groups of Lie type of characteristic
\ell and are cyclic groups.
B(i) The total
\ell-rank
\text{rk}_{\ell }\bar{{\rm\Gamma}}_{\ell } of
\bar{{\rm\Gamma}}_{\ell } (Definition 14) is equal to the rank of
\bar{\mathbf{S}}_{\ell } and is therefore independent of
\ell.
B(ii) The
A_{n}-type
\ell-rank
\text{rk}_{\ell }^{A_{n}}\bar{{\rm\Gamma}}_{\ell } of
\bar{{\rm\Gamma}}_{\ell } (Definition 14) for
n\in \mathbb{N}\setminus \{1,2,3,4,5,7,8\} and the parity of
(\text{rk}_{\ell }^{A_{4}}\bar{{\rm\Gamma}}_{\ell })/4 are independent of
\ell.