Can you tell me what power is greater?

Consider all integers n n which satisfy the following condition:

36 ! 0 ( m o d n ) 36! \equiv 0 \pmod{n} , but for all i < 36 i < 36 , we have i ! ≢ 0 ( m o d n ) i! \not \equiv 0 \pmod{n} .

The smallest positive integer value of n n that satisfies this condition can be written in the form a b a^b , where a a is a prime number. What is the value of a + b a + b ?


The answer is 19.

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1 solution

Mikael Marcondes
Jan 2, 2015

If the equation 36 ! 0 ( m o d 36! ≡ 0 (mod n ) n) becomes true only when we achieve the number 36! , this means that any divisors of n n are being "taken away" at each factor of this number, and the last divisor (or divisors) is (are) taken on 36 = 2 2 × 3 2 36=2^{2} \times{3^{2}} . The divisor taken on 36! only can be 2 or 3 . The number of times which the factor 2 appears between 1 and 35 , can be counted by the intervals. Hence, we have:

[ 2 , 34 ] 2 = 17 \frac{[2, 34]}{2}=17

[ 4 , 32 ] 4 = 8 \frac{[4, 32]}{4}=8

[ 8 , 32 ] 8 = 4 \frac{[8, 32]}{8}=4

[ 16 , 32 ] 16 = 2 \frac{[16, 32]}{16}=2

32 32 = 1 \frac{32}{32}=1

17 + 8 + 4 + 2 + 1 = 32 17+8+4+2+1=32

[ 3 , 33 ] 3 = 11 \frac{[3, 33]}{3}=11

[ 9 , 27 ] 9 = 3 \frac{[9, 27]}{9}=3

27 27 = 1 \frac{27}{27}=1

11 + 3 + 1 = 15 11+3+1=15

When 36! takes away any divisor, n becomes 2 33 2^{33} or 3 16 3^{16} . Now, we only need to find which power is smaller. Two different numbers can only be smaller than another, equal to another or greater than another. We can use a question mark as operator, as follows:

2 33 2^{33} ? 3 16 3^{16}

2 33 16 2^{\frac{33}{16}} ? 3 3

2 2 × 2 1 16 2^{2} \times {2^{\frac{1}{16}}} ? 3 3

2 1 16 2^{\frac{1}{16}} ? 3 4 \frac{3}{4}

As we know, any power of a number greater than one, is greater than one too. So, the left side of the comparation is greater than the right side.

2 33 2^{33} > 3 16 3^{16}

n = a b = 3 16 n=a^{b}=3^{16}

And the sum is a + b = 3 + 16 = 19 a+b=3+16=\boxed{19} .

Gah! Made a silly mistake. Counted the number of 3's in 36! and didn't realize that I could've used 3^16 instead of 3^17, so I entered 20. Cool problem!

Ryan Tamburrino - 6 years, 5 months ago

We can also see 3^16 is smaller because even 4^16=2^32 is smaller so 3^16 of course will be smaller.

Kushagra Sahni - 5 years ago

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