For m ∈ N and m ≤ 5 0 , let N = 1 ! ⋅ 2 ! ⋅ 3 ! ⋅ 4 ! ⋅ 5 ! ⋯ ( m − 2 ) ! ⋅ ( m − 1 ) ! ⋅ m !
The trailing number of zeros that N have is denoted by E .
Find the number of possible m for which E is prime.
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Thanks for the solution +1!. I think it was not a good idea for me to go till 50 numbers while framing the problem. The problem looks too harsh while solving. Great work.
It will be nice if someone find a generalization of the solution to the problem above.
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yup that is the major intention i posted the problem and the reason i dragged to 50 numbers. We can in fact write the recurring form using floor functions but ultimately we will have to count. And counting number of primes in list of 50 numbers is in itself a big task for which we've got to list down numbers. Though, the listing can be simplified by eliminating all evens and numbers divisible by 3 or by similar tricks since we know the difference between numbers.
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If we want we can write following obvious thing but it will not reduce the task. Instead for listing the above method is better
N ( m ) = N ( m − 1 ) + { ⌊ 5 m ! ⌋ + ⌊ 2 5 m ! ⌋ . . . . . . . . . . . . . . . . . ⌊ 5 k m ! ⌋ }
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Each factorial from 0 to 4 will add 0 trailing zeroes to N .
Each factorial from 5 to 9 will add 1 trailing zero to N , since each of these factorials is a multiple of 5 1 .
Each factorial from 1 0 to 1 4 will add 2 trailing zeroes to N , since each of these factorials is a multiple of 5 2 .
Each factorial from 1 5 to 1 9 will add 3 trailing zeroes to N , since each of these factorials is a multiple of 5 3 .
Each factorial from 2 0 to 2 4 will add 4 trailing zeroes to N , since each of these factorials is a multiple of 5 4 .
Each factorial from 2 5 to 2 9 will add 6 trailing zeroes to N , since each of these factorials is a multiple of 5 6 .
Each factorial from 3 0 to 3 4 will add 7 trailing zeroes to N , since each of these factorials is a multiple of 5 7 .
Each factorial from 3 5 to 3 9 will add 8 trailing zeroes to N , since each of these factorials is a multiple of 5 8 .
Each factorial from 4 0 to 4 4 will add 9 trailing zeroes to N , since each of these factorials is a multiple of 5 9 .
Each factorial from 4 5 to 4 9 will add 1 0 trailing zeroes to N , since each of these factorials is a multiple of 5 1 0 .
Factorial of 5 0 will add 1 2 trailing zeroes to N , since this factorial is a multiple of 5 1 2 .
So, the number of trailing zeroes ( E ) for each N will be:
From these possible values of E , only { 2 , 3 , 5 , 7 , 1 1 , 1 3 , 1 0 1 , 1 3 1 , 1 3 9 , 1 7 3 , 1 9 1 } are prime. So, 1 1 values are prime.