Keeping it 360

Find the first three digits of the smallest positive integer N N that has exactly 360 positive integer divisors.


The answer is 360.

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1 solution

Zee Ell
Nov 2, 2018

360 = 2 3 × 3 2 × 5 360 = 2^3×3^2×5

We can get the smallest suitable value by taking the product of the smallest primes and putting them to the smallest possible powers (in a way that the smallest prime number (2) is put to the power of the ((biggest prime factor of 360) - 1), then the next smallest prime (3) is put to the power of the ((second biggest prime factor of 360) - 1) and so on).

2 5 1 × 3 3 1 × 5 3 1 × 7 2 1 × 1 1 2 1 × 1 3 2 1 = 2 4 × 3 2 × 5 2 × 7 × 11 × 13 = 3603600 2^{5-1}×3^{3-1}×5^{3-1}×7^{2-1}×11^{2-1}×13^{2-1} = 2^4×3^2×5^2×7×11×13 = 3603600

Indeed, this number has:

( 4 + 1 ) ( 2 + 1 ) ( 2 + 1 ) ( 1 + 1 ) ( 1 + 1 ) ( 1 + 1 ) = 5 × 3 2 × 2 3 = 360 positive integer factors. (4+1)(2+1)(2+1)(1+1)(1+1)(1+1) = 5 × 3^2 × 2^3 = 360 \text { positive integer factors.}

(See relevant wiki: Factors )

It is easy, but a bit longer to prove that 3603600 is the smallest positive integer with exactly 360 positive divisors. One way to do that, is by trying to leave out prime factors and combining its power component with that of one of the smallest primes. This would result in bigger numbers, because e.g.:

2 5 1 × 1 3 2 1 < 2 5 × 2 1 and 2 5 1 × 3 3 1 < 2 5 × 3 1 and 3 3 1 × 1 3 2 1 < 3 3 × 2 1 2^{5-1}×13^{2-1} < 2^{5×2-1} \text { and } 2^{5-1}×3^{3-1} < 2^{5×3-1} \text { and } 3^{3-1}×13^{2-1} < 3^{3×2-1}

Hence the first three digits are: 360 \text {Hence the first three digits are: } \boxed {360}

Very nicely explained! Thank you!

As you say, the key observation is that the exponents n k n_k in the prime factorization N = 2 n 1 3 n 2 5 n 3 . . . N=2^{n_1}3^{n_2}5^{n_3}... must form a nonincreasing sequence with ( n k + 1 ) = 360 \prod(n_k+1)=360 (otherwise we could swap them to construct a smaller integer with the same number of divisors). Once we realise that, there are very few cases left to check.

Otto Bretscher - 2 years, 7 months ago

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When I first saw this problem, I really wanted to guess 360(because the title), but when I solved it, I realized guessing might be faster:p

X X - 2 years, 7 months ago

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Even better, we have two 360's in a row (because of the factor 1001 = 7 × 11 × 13 1001=7\times 11 \times 13 ;)

Otto Bretscher - 2 years, 7 months ago

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@Otto Bretscher Yeah, so beautiful!!

X X - 2 years, 7 months ago

I must say this is nice problem that is related to Superabundant numbers .

Naren Bhandari - 2 years, 7 months ago

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