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I know that for general linear group $\mathrm{GL}(n,p^r)$, one Sylow $p$-subgroup is the set of all unitriangular matrices. I need a reference for this theorem. Thank you.

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    $\begingroup$ This is proved in Herstein's book "Topics in Algebra" for example-maybe second or third edition. Once you know the order of ${\rm GL}$,it is a matter of verifying that the index of the subgroup of upper unitriangular matrices is prime to $p,$ which is clear. $\endgroup$ May 11, 2013 at 9:42
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    $\begingroup$ You can find this as Example 2.1 of the beautiful <a href="arxiv.org/pdf/math/0503154v6.pdf"> small book by J.-P. Serre </a> on finite groups. $\endgroup$ May 11, 2013 at 10:42
  • $\begingroup$ For the Sylow-group theory, I like Hungerford's deployment very much. $\endgroup$
    – Henry.L
    May 11, 2013 at 10:50
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    $\begingroup$ @unknown: This is far from a research-level question, being well-known for generations and written down in textbooks. All it requires it the easy computation of the group and subgroup orders. Did you try first at math.stackexchange.com? $\endgroup$ May 11, 2013 at 13:20
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    $\begingroup$ @unknown: In a research paper, I'd probably just refer to this basic example (it's not a real theorem) as "well known". But if pressed to supply a published reference, I'd still emphasize the elementary nature of the example by citing Exercise 8.9 in J.-P. Serre, Linear Representations of Finite Groups, Springer, 1977 (English translation of an earlier French edition). The computation of group and subgroup orders here is straightforward, as other suggested references indicate. $\endgroup$ May 13, 2013 at 20:12

2 Answers 2

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Steinberg's "Lecture Notes on Chevalley Groups" the Corollary of Lemma 54 on page 132. There is possibly a more modern reference.

EDIT: Sorry the reference to Steinberg is not sufficient as he does not treat arbitrary finite reductive groups. However all that is needed is to obtain a slightly more general formula for the order of an arbitrary finite reductive group. This is obtained, for instance, in Corollary 4.2.5 of Geck's book "An Introduction to Algebraic Geometry and Algebraic Groups". the important point is a theorem of Rosenlicht about the fixed points of a connected unipotent group under a Frobenius endomorphism. If $\mathbf{U}$ is a connected unipotent affine algebraic group with Frobenius endomorphism $F' : \mathbf{U} \to \mathbf{U}$ then the finite group $\mathbf{U}^{F'}$ has order $q^{\dim\mathbf{U}}$, (see Geck - Theorem 4.2.4).

Now take $\mathbf{G} = \mathrm{GL}_n(\overline{\mathbb{F}}_p)$ and $F$ to be the map given by $F(x_{ij}) = (x_{ij}^q)$ where $q = p^a$ for some integer $a$ then $\mathbf{G}^F = \mathrm{GL}_n(q)$. The subgroup $\mathbf{B} \leqslant \mathbf{G}$ of upper triangular matrices is an $F$-stable Borel subgroup of $\mathbf{G}$ and its unipotent radical $\mathbf{U} \leqslant \mathbf{B}$ is the subgroup of all upper uni-triangular matrices. This has maximal dimension amongst all connected unipotent subgroups of $\mathbf{G}$, which can be seen in the following way. Assume $\mathbf{V} \leqslant \mathbf{G}$ is a connected unipotent subgroup of $\mathbf{G}$ then as $\mathbf{V}$ is solvable it is contained in a Borel subgroup $\mathbf{B}'$ of $\mathbf{G}$. As $\mathbf{V}$ is a unipotent subgroup of $\mathbf{B}'$ it is contained in the unipotent radical of $\mathbf{B}'$ and as all Borel subgroups of $\mathbf{G}$ are conjugate we have the unipotent radicals of $\mathbf{B}$ and $\mathbf{B}'$ have the same dimension. In particular, we have $\dim\mathbf{V} \leqslant \dim \mathbf{U}$. Now applying Rosenlicht's theorem we see that the order of $\mathbf{U}^F$ is a power of $p$. We can now apply Steinberg's argument to the order formula for finite reductive groups to deduce that the index of $\mathbf{U}^F$ in $\mathbf{G}^F$ is coprime to $p$. This approach has the advantage that it shows that the fixed points of the unipotent radical of any $F$-stable Borel subgroup of $\mathbf{G}$ is a Sylow $p$-subgroup of $\mathbf{G}^F$.

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  • $\begingroup$ Note that as Geoff points out in the case of $\mathrm{GL}_n(q)$ this is quite easy to deduce from the order formula for $\mathrm{GL}_n(q)$, which is quite easy to work out without the theory of algebraic groups. $\endgroup$
    – Jay Taylor
    May 11, 2013 at 10:07
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This is in the book Bogopolski, Oleg, Introduction to group theory. Translated, revised and expanded from the 2002 Russian original. EMS Textbooks in Mathematics. European Mathematical Society (EMS), Zürich, 2008. x+177 pp. Bogopolski uses it to prove the Sylow theorems.

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