Abstract
Given a compatible subsystem \(\{\rho _\ell \}_\ell \) of n-dimensional \(\ell \)-adic Galois representations arising from étale cohomology of any complete, non-singular variety over a number field K, we define \(\Gamma _\ell := \rho _\ell ({\text {Gal}}(\bar{K}/K))\) and let \(\mathbf {G}_\ell \) denote the Zariski closure of \(\Gamma _\ell \) in \(\mathrm {GL}_n\). If \(\mathbf {G}_\ell \) is of Type A in the sense that all simple composition factors are of type A in the Cartan-Killing classification, then \(\Gamma _\ell \) is, in a suitable sense, maximal in \(\mathbf {G}_\ell \) for all \(\ell \gg 0\). As a corollary, if \(\rho _\ell \) is semisimple and \(\ell \) is sufficiently large, then \(\mathbf {G}_\ell \) is unramified.
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We would like to thank Frank Calegari for his comments on an earlier draft of this paper.
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Chun Yin Hui is supported by the National Research Fund, Luxembourg, and cofunded under the Marie Curie Actions of the European Commission (FP7-COFUND). Michael Larsen was partially supported by a grant from the National Science Foundation.
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Hui, C.Y., Larsen, M. Type A images of Galois representations and maximality. Math. Z. 284, 989–1003 (2016). https://doi.org/10.1007/s00209-016-1683-0
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DOI: https://doi.org/10.1007/s00209-016-1683-0