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The following question arises while I am reading a paper of B. N. Cooperstein.

In the Table 1 of his paper, two groups have smaller permutation degree compared to other members in their family: $\mathrm{PSp}_4(3)$ with permutation degree 27 and $\mathrm{PSU}_3(5)$ with permutation degree 50, while $\mathrm{PSp}_{2n}(q)$ generally has a permutation degree of $$\frac{q^{2n}-1}{q-1}$$ and $\mathrm{PSU}_3(q)$ has a permutation degree of $q^3+1$. It is already known to Camille Jordan that $\mathrm{PSp}_4(3)$ is a permutation group on 27 lines of a cubic surface. I am curious if there is a similar geometric configuration for $\mathrm{PSU}_3(5)$.

Question: Is there any neat geometric configuration (with 50 elements) that the group $\mathrm{PSU}_3(5)$ is a permutation group which acts faithfully on the configuration?

Edit: A table for permutation degree of simple groups can be found here.

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For me the article by B. N. Cooperstein is behind a paywall. But I think the object you are looking for is the Hoffman–Singleton graph. There is a wikipedia page about that graph, see

https://en.wikipedia.org/wiki/Hoffman%E2%80%93Singleton_graph

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  • $\begingroup$ Welcome to mathoverflow. Why don't you add a link to the wikipedia page? $\endgroup$
    – Sebastian Goette
    Commented Mar 4, 2017 at 18:40
  • $\begingroup$ You are right, I have added the link. It just took me some time to find out how to do it. $\endgroup$
    – Martin Seysen
    Commented Mar 4, 2017 at 19:32
  • $\begingroup$ perhaps it is also worth mentioning that the degree 27 action of Sp(4,3) arises from its isomorphism to $U(4,2)$, for which the action of degree 27 is the action on totally isotropic lines in its natural 4-dimensional module over GF(4). In the case of degree 50 action of U(3,5), nothing like this happens. $\endgroup$
    – Dima Pasechnik
    Commented Mar 4, 2017 at 19:51
  • $\begingroup$ I found this table math.wisc.edu/~grizzard/papers/table.pdf just now. I will put the link in my question. $\endgroup$
    – Y. Zhao
    Commented Mar 4, 2017 at 20:29

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