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One and only (Posted on 2016-04-28) Difficulty: 2 of 5
There is only one prime p such that p! is p digits long.

Find it and prove its uniqueness.

No Solution Yet Submitted by Ady TZIDON    
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spoiler | Comment 2 of 4 |
25!=15511210043330985984000000 which is 25 digits long.

p! = p*(p-1)!.  Then for p>=10, p! has at least one more digit than (p-1)!.

For p<25, only 23 works:  23! = 25852016738884976640000.

  Posted by xdog on 2016-04-28 09:52:53
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