Prime Trailing Zeros

For m N m\in\Bbb N and m 50 m\le 50 , let N = 1 ! 2 ! 3 ! 4 ! 5 ! ( m 2 ) ! ( m 1 ) ! m ! \large N = 1! \cdot 2! \cdot 3! \cdot 4! \cdot 5! \cdots (m-2)! \cdot (m-1)! \cdot m!

The trailing number of zeros that N N have is denoted by E E .

Find the number of possible m m for which E E is prime.


Inspiration .


The answer is 11.

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1 solution

Guilherme Niedu
May 15, 2017

Each factorial from 0 0 to 4 4 will add 0 0 trailing zeroes to N N .

Each factorial from 5 5 to 9 9 will add 1 1 trailing zero to N N , since each of these factorials is a multiple of 5 1 5^1 .

Each factorial from 10 10 to 14 14 will add 2 2 trailing zeroes to N N , since each of these factorials is a multiple of 5 2 5^2 .

Each factorial from 15 15 to 19 19 will add 3 3 trailing zeroes to N N , since each of these factorials is a multiple of 5 3 5^3 .

Each factorial from 20 20 to 24 24 will add 4 4 trailing zeroes to N N , since each of these factorials is a multiple of 5 4 5^4 .

Each factorial from 25 25 to 29 29 will add 6 6 trailing zeroes to N N , since each of these factorials is a multiple of 5 6 5^6 .

Each factorial from 30 30 to 34 34 will add 7 7 trailing zeroes to N N , since each of these factorials is a multiple of 5 7 5^7 .

Each factorial from 35 35 to 39 39 will add 8 8 trailing zeroes to N N , since each of these factorials is a multiple of 5 8 5^8 .

Each factorial from 40 40 to 44 44 will add 9 9 trailing zeroes to N N , since each of these factorials is a multiple of 5 9 5^9 .

Each factorial from 45 45 to 49 49 will add 10 10 trailing zeroes to N N , since each of these factorials is a multiple of 5 10 5^{10} .

Factorial of 50 50 will add 12 12 trailing zeroes to N N , since this factorial is a multiple of 5 12 5^{12} .

So, the number of trailing zeroes ( E E ) for each N N will be:

N E
1 0
2 0
3 0
4 0
5 1
6 2
7 3
8 4
9 5
10 7
11 9
12 11
13 13
14 15
15 18
16 21
17 24
18 27
19 30
20 34
21 38
22 42
23 46
24 50
25 56
26 62
27 68
28 74
29 80
30 87
31 94
32 101
33 108
34 115
35 123
36 131
37 139
38 147
39 155
40 164
41 173
42 182
43 191
44 200
45 210
46 220
47 230
48 240
49 250
50 262

From these possible values of E E , only { 2 , 3 , 5 , 7 , 11 , 13 , 101 , 131 , 139 , 173 , 191 } \{ 2, 3, 5, 7, 11, 13, 101, 131, 139, 173, 191 \} are prime. So, 11 \color{#3D99F6} \boxed{11} values are prime.

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.

Mayank Chaturvedi - 4 years ago

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Thanks, my friend!

Guilherme Niedu - 4 years ago

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.

Mayank Chaturvedi - 4 years ago

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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 ) + { m ! 5 + m ! 25 . . . . . . . . . . . . . . . . . m ! 5 k } N(m)=N(m-1)+\left\{ \left\lfloor \frac { m! }{ 5 } \right\rfloor +\left\lfloor \frac { m! }{ 25 } \right\rfloor .................\left\lfloor \frac { m! }{ { 5 }^{ k } } \right\rfloor \right\}

Mayank Chaturvedi - 4 years ago

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