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$\DeclareMathOperator\PSL{PSL}\DeclareMathOperator\PSU{PSU}$Cross-post from MSE. There are some very interesting comments on the original post if you want to go check it out.

Are there any well known patterns about which finite simple groups have order $ p+1 $ for $ p $ a prime?

Here is a list of all non-cyclic simple groups of order up to 100,000 and whether they have order p+1 (there are 31 such groups, 16 have order $ p+1 $)

  • $ \PSL_2(5) $, $p=59$

  • $ \PSL_2(7) $, $p=167$

  • $ \PSL_2(9) $, $p=359$

  • $ \PSL_2(8) $, $p=503$

  • $ \PSL_2(11) $, $p=659$

  • $ \PSL_2(13) $, $p=1091$

  • $ \PSL_2(17) $, $p=2447$

  • $ A_7 $, $2519$ not prime

  • $ \PSL_2(19) $, $3419$ not prime

  • $ \PSL_2(16) $, $p=4079$

  • $ \PSL_3(3) $, $5615$ not prime

  • $ PSU_3(3) $, $p=6047$

  • $ \PSL_2(23) $, $6071$ not prime

  • $ \PSL_2(25) $, $7799$ not prime

  • $ M_{11} $, $p=7919$

  • $ \PSL_2(27) $, $9827$ not prime

  • $ \PSL_2(29) $, $12{,}179$ not prime

  • $ \PSL_2(31) $, $p=14{,}879$

  • $ \PSL_4(2) $, $20{,}159$ not prime

  • $ \PSL_3(4) $, $20{,}159$ not prime

  • $ \PSL_2(37) $, $p=25{,}307$

  • $ \PSU_4(2) $, $p=25{,}919$

  • $ \operatorname{Suz}(8) $, $29{,}119$ not prime

  • $ \PSL_2(32) $, $32{,}735$ not prime

  • $ \PSL_2(41) $, $p=34{,}439$

  • $ \PSL_2(43) $, $p=39{,}731$

  • $ \PSL_2(47) $, $51{,}887$ not prime

  • $ \PSL_2(49) $, $58{,}799$ not prime

  • $ \PSU_3(4) $, $62{,}399$ not prime

  • $ \PSL_2(53) $, $p=74{,}411$

  • $ M_{12} $, $95{,}039$ not prime.

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    $\begingroup$ Any particularly interesting comments you'd like to single out? \\ TeX note: in math mode, you must use braces to turn off the special spacing around commas (intended for cases like the ordered pair $(74, 411)$). Thus, you must write something like $74{,}411$ 74{,}411, not $74,411$ 74,411. I have edited accordingly. $\endgroup$
    – LSpice
    Commented Sep 26, 2022 at 14:40
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    $\begingroup$ @LSpice oh thank you, I'll make sure to do that in the future! $\endgroup$ Commented Sep 26, 2022 at 14:42
  • $\begingroup$ See here: mathoverflow.net/questions/48618/… $\endgroup$
    – spin
    Commented Sep 27, 2022 at 5:07
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    $\begingroup$ @LSpice one interesting comment points out that OEIS has a sequence for all $ A_n $ whose order is $ p+1 $. Looks like this OEIS entry was also mentioned in the post linked to by @ spin. Another cool comment mentions that $ 40{,}007 $ of the $ 403 {,} 864 $ orders of finite simple groups less than $ 10^{20} $ are of the form $ p+1 $ $\endgroup$ Commented Sep 27, 2022 at 13:06
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    $\begingroup$ @IanGershonTeixeira: I think it is better not to delete questions. This could be closed as a duplicate, but even that is not really necessary since the questions are linked. $\endgroup$
    – spin
    Commented Sep 28, 2022 at 6:20

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