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The only solution would be going through all cycle types of all permutations which is a lot of work. Is there any smarter solution than this one?

Thank you in advance!

Diglett
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The maximum order of an element of $S_n$ is assumed for a permutation which is a product of cycles of distinct lengths. So in stead of considering all cycle types, you only need to consider the cycle types $$(1,2,4),\qquad (1,6),\qquad(2,5),\qquad(3,4),\qquad(7),$$ which quickly shows that the maximum order is $12$.

In fact more is true; the maximum is assumed for a cycle type of which the entries are pairwise coprime prime powers, where $1$ also counts as a prime power. This leaves only the latter three cycle types above, and for larger symmetric groups this reduces the number of cycle types to check significantly. This is harder to prove though.

Edit: To clarify, the sequences above are not cycles in $S_7$, but cycle types of elements of $S_7$. For example, the cycle type of $(2\ 3)(4\ 5\ 6\ 7)\in S_7$ is $(1,2,4)$, because it is the product of a $1$-cycle, a $2$-cycle and a $4$-cycle.

Servaes
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    Triviality: you need to consider $(7)$ as well. – ancient mathematician Apr 12 '19 at 16:04
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    @Servaes: Could you please explain me why I do not have to consider, for instance, the cycle type $(2,2,3)$? How can I deduce this case from you cases? – Diglett Apr 12 '19 at 16:34
  • @ancientmathematician You are very right. – Servaes Apr 12 '19 at 16:52
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    @Diglett If two cycles have the same length, you can replace them by one cycle of twice the length, and the order will not decrease. In your example $\operatorname{lcm}{2,2,3}\leq\operatorname{lcm}{4,3}$. – Servaes Apr 12 '19 at 16:53
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    What does "type" mean here? 3 and 1 cycles I get... but why are there three random 2-cycles? I think i'm probably just missing something simple, but what eliminates 4-5-6-7 cycles at the start (besides inspection?) – rschwieb Apr 12 '19 at 17:46
  • @rschwieb I'm used to the terms of cycles being written without comma's, separated only by spaces. So for example $(1\ 2)$ and $(3\ 4\ 7)$ are cycles in $S_7$. A cycle type of an element of $S_7$ is a sequence $(c_1,\ldots,c_n)$ of positive integers with $\sum c_i=7$. So the cycle type of $(1\ 2)(3\ 4\ 7)$ is $(3,2,1,1)$. I'll edit my answer to clarify. – Servaes Apr 12 '19 at 19:29
  • @Servaes OH, yeah I thought you were talking about cycle notation. That was my problem – rschwieb Apr 13 '19 at 02:15