Fun facts about prime numbers
These numbers are all primes:

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31
331
3331
33331
333331
3333331
33333331
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1234567891
908070601
9080706050401
1231231237 -- ("123" repeated 3 times, then 7)
1481481481 -- ("148" repeated 3 times, then 1)
6666666661
9876543211
9876543217
9999999929
11111111113 -- ("1" repeated 10 times, then 3)
31111111111 -- ("3", then "1" repeated 10 times)
13131313133 -- ("13" repeated 5 times, then 3)
44444444441 -- ("4" repeated 10 times, then 1)
66666666667 -- ("6" repeated 10 times, then 7)
28282828283 -- ("28" repeated 5 times, then 3)
98989898981 -- ("98" repeated 5 times, then 1)
377777777777 -- ("3", then "7" repeated 11 times)
444444444443 -- ("4" repeated 11 times, then 3)
555555555551 -- ("5" repeated 11 times, then 1)
888888888887 -- ("8" repeated 11 times, then 7)
5555555555551 -- ("5" repeated 12 times, then 1)
7777777777771 -- ("7" repeated 12 times, then 1)
888888888888883 -- ("8" repeated 14 times, then 3)
222222222222222221 -- ("2" repeated 17 times, then 1)
1111111111111111111 -- ("1" repeated 19 times)
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11!+1 = 39916801

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