User mikko korhonen - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-18T22:05:56Z http://mathoverflow.net/feeds/user/10146 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/109027/applications-of-frobenius-theorem-and-conjecture Applications of Frobenius theorem and conjecture Mikko Korhonen 2012-10-06T23:10:39Z 2013-05-27T09:43:59Z <p>A theorem of Frobenius states that if $n$ divides the order of a finite group $G$, then the number of solutions to $x^n = 1$ in $G$ is a multiple of $n$. Frobenius conjectured that if the number of solutions is exactly $n$, then the set of solutions form a characteristic subgroup of $G$. The conjecture was eventually proved in the 90's, and the full proof uses the classification of finite simple groups.</p> <p>The theorem feels a bit isolated for me.. I'm not sure how the conjecture fits into a wider context, either. What is their importance, if any? Are there any good examples of applications of the theorem or the conjecture? If I'm interested in finite groups, why should I care about the theorem or the conjecture, other than that they are kind of neat?</p> <p>One example I know is that if $G$ has every Sylow subgroup cyclic, then with Frobenius theorem we can show that the Sylow subgroup corresponding to the largest prime divisor of $G$ is normal. Also (this one is too easy, but I like it) for any prime $p$, the number of elements satisfying $x^p = 1$ in the symmetric group $S_p$ is $(p-1)! + 1$, so Frobenius theorem implies $(p-1)! \equiv -1 \mod p$. </p> http://mathoverflow.net/questions/119421/lower-bounds-on-the-number-of-elements-in-sylow-subgroups Lower bounds on the number of elements in Sylow subgroups Mikko Korhonen 2013-01-20T20:09:45Z 2013-03-05T19:22:00Z <p>I posted this question on Math.SE (<a href="http://math.stackexchange.com/questions/278600/lower-bounds-on-the-number-of-elements-in-sylow-subgroups" rel="nofollow">link</a>), but it didn't get any answers so I'm going to ask here. This is an edited version of the question.</p> <hr> <p>Let $p$ be a prime and $n \geq 1$ some integer. Furthermore, let $G$ be a finite group where $p$-Sylow subgroups have order $p^n$. Denote by $n_p(G)$ the number of Sylow $p$-subgroups of $G$. Denote the number of elements in the union of all Sylow $p$-subgroups of $G$ by $f_p(G)$. I am interested in finding lower bounds for $f_p(G)$ that do not depend on the group $G$, but only on $p$, $n$ and $n_p(G)$.</p> <p>By Sylow's theorem, we know that $n_p(G) = kp + 1$ for some integer $k \geq 0$. What I know so far:</p> <ul> <li>If $k = 0$, then $f_p(G) = p^n$.</li> <li>If $k = 1$, then $f_p(G) = p^{n+1}$.</li> <li>If $k \geq 2$, then $f_p(G) \geq 2p^{n+1} - p^n$.</li> </ul> <p>This is a theorem due to G. A. Miller, see also <a href="http://math.stackexchange.com/questions/233071/if-a-finite-group-has-geq-p1-sylow-p-subgroups-of-order-pn-then-ther" rel="nofollow">this question</a> from Math.SE. To prove the inequality in the case $k \geq 2$, you first prove that then $f_p(G) > p^{n+1}$. Then observe that $f_p(G) - 1$ is divisible by $p-1$, then the inequality follows from Frobenius theorem (*). Details are in a book of Miller, Blichfeldt and Dickson (Theory and Applications of Finite Groups) and a paper of Miller ("Some deductions from Frobenius Theorem").</p> <p>My main question is the following:</p> <blockquote> <p>What is a better lower bound for the case $k > 2$?</p> </blockquote> <p>The case $n = 1$ is easy, because then we know the value of $f_p(G)$ precisely. If $n = 1$, then $f_p(G) = n_p(G)(p-1)+1$. What about when $n > 1$? Answers regarding particular $n$ or particular $k$ are also welcome. </p> <p>If the Sylow $p$-subgroups are cyclic, then we have $f_p(G) \geq n_p(G)\varphi(p^n) + p^{n-1}$ and this bound is okay. But most $p$-groups are not cyclic..</p> <p>I think the following example shows that $f_p(G)$ gets arbitrarily large values for fixed $p$ and $n$ (not surprising). By Dirichlet's theorem, there exist arbitrarily large primes $q$ such that $q \equiv 1 \mod{p}$. Then in a direct product $G = C_{p^{n-1}} \times H$, where $H$ is a non-abelian group of order $pq$, the Sylow subgroups of $G$ have $C_{p^{n-1}}$ as their common intersection. There are exactly $q$ Sylow $p$-subgroups, because otherwise $G$ would be nilpotent but its subgroup $H$ is not. Therefore the number of elements in the $p$-Sylow subgroups is $f_p(G) = q(p^{n} - p^{n-1}) + p^{n-1}$, and this goes to infinity as $q$ goes to infinity. Thus there exist groups $G$ with Sylow $p$-subgroups of order $p^n$ such that $f_p(G)$ is arbitrarily large.</p> <p>Also, $f_p(G) \rightarrow \infty$ as $k \rightarrow \infty$. This is seen by noticing that $f_p(G)^{p^n} \geq n_p(G)$, so </p> <p>$$f_p(G) \geq (kp + 1)^{p^{-n}}$$</p> <p>which goes to infinity as $k \rightarrow \infty$.</p> <p>One more observation: not all integers $\equiv 1 \mod{p}$ are possible amounts of Sylow $p$-subgroups. For example, there does not exist a group with exactly $22$ Sylow $3$-subgroups, although $22 \equiv 1 \mod{3}$. I don't know if this complicates things.</p> <p>(*) Frobenius Theorem says that when $G$ is a finite group with order divisible by $s$, the number of solutions to $x^s = 1$ in $G$ is a multiple of $s$. We know that $f_p(G)$ is the number of solutions to $x^{p^n} = 1$ in $G$.</p> http://mathoverflow.net/questions/109027/applications-of-frobenius-theorem-and-conjecture/122025#122025 Answer by Mikko Korhonen for Applications of Frobenius theorem and conjecture Mikko Korhonen 2013-02-16T21:48:23Z 2013-02-16T21:48:23Z <p>Since nobody gave any examples of applications of Frobenius conjecture, here's a small one I read about recently. We will only consider finite groups in what follows. For a group $G$, denote the set of elements of $G$ satisfying $x^n = 1$ by $a_n(G)$.</p> <p>If for each positive integer $n$, the groups $G$ and $H$ have the same number of elements of order $n$, we say that $G$ and $H$ have the same order structure. It is not hard to see that $G$ and $H$ have the same order structure if and only if $a_n(G) = a_n(H)$ for every integer $n$. </p> <p><strong>Question:</strong> if $H$ has the same order structure as $G$, how does the structure of $G$ affect the structure of $H$? It is clear that we will have $|G| = |H|$. We cannot say that $G$ and $H$ must be isomorphic: take $G$ and $H$ to be $p$-groups both of same order, both of exponent $p$, $G$ abelian, $H$ nonabelian. So $G$ abelian does not imply $H$ abelian. However, $G$ cyclic does imply that $H$ must be cyclic. We can also show that $G$ nilpotent implies that $H$ is nilpotent. It has been proven (using CFSG) that if $G$ is simple, then $G$ and $H$ must be isomorphic if they have the same order structure. </p> <p>There is an open problem (due to Thompson, I think) that asks the following question:</p> <blockquote> <p><strong>Problem:</strong> Suppose that $G$ is solvable and that $G$ and $H$ have the same order structure. Does this imply that $H$ is solvable?</p> </blockquote> <p>Using Frobenius conjecture, we can deduce a partial result:</p> <blockquote> <p><strong>Theorem:</strong> If $G$ is supersolvable and $G$ and $H$ have the same order structure, then $H$ is solvable.</p> </blockquote> <p><strong>Proof:</strong> Suppose $G$ has order $|G| = p_1^{a_1} \ldots p_t^{a_t}$, where $p_1 &lt; p_2 &lt; \ldots &lt; p_t$ are primes. We know that $|G| = |H|$. Furthermore, since $G$ supersolvable, we see that $a_n(G) = n$ for every $n = p_i^{a_i} p_{i+1}^{a_{i+1}} \ldots p_t^{a_t}$. Hence $a_n(H) = n$ for every such $n$ since $G$ and $H$ have the same order structure. By Frobenius conjecture, $H$ has a unique subgroup of order $p_i^{a_i} p_{i+1}^{a_{i+1}} \ldots p_t^{a_t}$ for every $i$ and thus $H$ must be solvable.</p> http://mathoverflow.net/questions/119421/lower-bounds-on-the-number-of-elements-in-sylow-subgroups/120899#120899 Answer by Mikko Korhonen for Lower bounds on the number of elements in Sylow subgroups Mikko Korhonen 2013-02-05T19:04:34Z 2013-02-05T19:04:34Z <p>This is just a small partial result and some comments. I think the following should settle the case $k = 2$.</p> <p>Suppose that $G$ is a group with Sylow $p$-subgroups of order $p^n$ and that $n_p(G) = 2p + 1$. According to a theorem of Marshall Hall (see theorem 3.1 in [*]), this can only happen if $2p + 1 = q^t$ is a power of a prime, so let's assume that this is the case.</p> <p>First of all, the lower bound $p^n(2p - 1)$ given in the question is attained. Let </p> <p>$$G = C_{p^{n-1}} \times AGL(1, q^t),$$</p> <p>where $AGL(1, q^t)$ is the group of invertible affine transformations $x \mapsto ax + b$ of the field of order $q^t$. Here $G$ has exactly $2p + 1$ Sylow $p$-subgroups, the Sylow $p$-subgroups have order $p^n$ and $f_p(G) = p^n(2p - 1)$.</p> <p>Now by Frobenius theorem $f_p(G) = tp^n$ where $t$ is an integer. Since $2p - 1 \leq t &lt; 2p + 1$, we see that $t = 2p - 1$ or $t = 2p$.</p> <p>If $p \neq 2$, then $t = 2p - 1$ because $t-1$ must be a multiple of $p - 1$ [**]. Therefore in this case $f_p(G) = p^n(2p - 1)$.</p> <p>If $p = 2$ and $n \geq 2$, then $f_2(G) = 2^{n+2}$ is attained by a semidirect product $G = C_{2^n} \ltimes_\theta C_5$ (I think, I'll check this later).</p> <hr> <p>I have not made much progress for the cases where $n_p(G) = kp + 1$ and $k > 2$. In the case where $n_p(G) = 3p + 1$, a theorem of Marshall Hall (see theorem 3.2 in [*]) shows that $p = 2$, $p = 3$ or $p = 5$. It seems that things get messy from now on with this approach, perhaps it's best to disregard "impossible values" like $n_3(G) = 22$. Or we could start with the case $p = 2$ where there are no impossible values. </p> <p>[*] M. Hall, On the number of Sylow subgroups in a finite group (1967) <a href="http://dx.doi.org/10.1016/0021-8693(67)90076-2" rel="nofollow">DOI link</a></p> <p>[**] Proof: Now $f_p(G) - 1$ is the number of elements of order $p^k$, where $1 \leq k \leq n$. Since the number of elements of order $s$ is always a multiple of $\varphi(s)$, we get that $f_p(G) - 1$ must be a multiple of $p-1$. Thus $t-1$ is also a multiple of $p-1$.</p> http://mathoverflow.net/questions/119421/lower-bounds-on-the-number-of-elements-in-sylow-subgroups Comment by Mikko Korhonen Mikko Korhonen 2013-01-23T11:02:53Z 2013-01-23T11:02:53Z @Nick: Ok, I'll edit the post. What I mean by the construction is this. Fix a prime $p$ and integer $n \geq 1$. The construction shows that we can find a group $G$ with Sylow $p$-subgroups of order $p^n$ (not just some arbitrary power of $p$ like in your comment) such that $f_p(G)$ is arbitrarily large. http://mathoverflow.net/questions/119421/lower-bounds-on-the-number-of-elements-in-sylow-subgroups Comment by Mikko Korhonen Mikko Korhonen 2013-01-21T13:21:34Z 2013-01-21T13:21:34Z Perhaps one way to start with this is that given $n$, $p$ and $k$, find the smallest possible value for $f_p(G)$. I don't know if the bound $2p^{n+1}−p^n$ is sharp for the case $k=2$. It is sharp for all primes $p$ such that $2p+1$ is prime, which can be seen by the construction in my question. http://mathoverflow.net/questions/119421/lower-bounds-on-the-number-of-elements-in-sylow-subgroups Comment by Mikko Korhonen Mikko Korhonen 2013-01-21T12:33:19Z 2013-01-21T12:33:19Z @Nick Gill: Yes, the lower bound will of course depend on $p$ and $n$, just like the lower bound given by Miller's thm does. Basically given $p$ and $n$, we're looking at functions $g$ such that $f_p(G) \geq g(k)$ for any finite group $G$ with Sylow subgroups of order $p^n$ and $n_p(G) = kp + 1$. Is this what you meant?