Abstract
Let p>5 be a prime. Let X be the reduction of the modular curve X(p) in characteristic ℓ (with ℓ≠p). Aside from two known cases in characteristic ℓ=3 (with p=7, 11), we show that the full automorphism group of X is PSL(2,p).
For John Thompson on the occasion of his seventieth birthday
The third author gratefully acknowledges the support of the NSF grant DMS-0140578.
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Bending, P., Camina, A., Guralnick, R. (2005). Automorphisms of the Modular Curve. In: Voelklein, H., Shaska, T. (eds) Progress in Galois Theory. Developments in Mathematics, vol 12. Springer, Boston, MA. https://doi.org/10.1007/0-387-23534-5_2
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DOI: https://doi.org/10.1007/0-387-23534-5_2
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