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Number theory Difficulty 6.3 National olympiad Find the answer

Determine all positive integers nn with at least 44 factors such that nn is the sum the squares of its 44 smallest factors.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Let d1,d2,d3,d4 d_1, d_2, d_3, d_4 be the smallest four positive divisors of n n . We know that d1=1 d_1 = 1 .
2. Given that n n is the sum of the squares of its four smallest factors, we have:
n=d12+d22+d32+d42 n = d_1^2 + d_2^2 + d_3^2 + d_4^2
Substituting d1=1 d_1 = 1 , we get:
n=12+d22+d32+d42=1+d22+d32+d42 n = 1^2 + d_2^2 + d_3^2 + d_4^2 = 1 + d_2^2 + d_3^2 + d_4^2
Therefore:
n1=d22+d32+d42 n - 1 = d_2^2 + d_3^2 + d_4^2
3. If n n were odd, then d1,d2,d3,d4 d_1, d_2, d_3, d_4 would all be odd, making n n even, which is a contradiction. Hence, n n must be even, implying d2=2 d_2 = 2 .
4. Substituting d2=2 d_2 = 2 into the equation, we get:
n5=d32+d42 n - 5 = d_3^2 + d_4^2
5. Since n n is even, d32+d42 d_3^2 + d_4^2 must be odd. This means one of d3 d_3 and d4 d_4 is odd while the other is even.

Case 1: If d4 d_4 is even, say d4=2m d_4 = 2m :
- If m=2 m = 2 , then d3 d_3 must be 3, and:
n=12+22+32+42=1+4+9+16=30 n = 1^2 + 2^2 + 3^2 + 4^2 = 1 + 4 + 9 + 16 = 30
However, d4=4 d_4 = 4 does not divide 30, so n=30 n = 30 is not a solution.
- If m>2 m > 2 , since m m must be a divisor (as 2m 2m is a divisor), d3=m d_3 = m and:
n=5+5m2 n = 5 + 5m^2
Since mn m \mid n :
m5+5m2    m5    m=5 m \mid 5 + 5m^2 \implies m \mid 5 \implies m = 5
Therefore:
n=5+552=5+125=130 n = 5 + 5 \cdot 5^2 = 5 + 125 = 130
We need to check if 130 satisfies the problem's conditions. The divisors of 130 are 1, 2, 5, 10, 13, 26, 65, and 130. The smallest four divisors are 1, 2, 5, and 10. We check:
12+22+52+102=1+4+25+100=130 1^2 + 2^2 + 5^2 + 10^2 = 1 + 4 + 25 + 100 = 130
Thus, n=130 n = 130 satisfies the conditions.

Case 2: If d3 d_3 is even, and there were some m3 m \geq 3 such that d3=2m d_3 = 2m :
- Then m m would be a divisor between d2 d_2 and d3 d_3 , which is a contradiction. So m2 m \leq 2 .
- Since m=1 m = 1 yields d3=2 d_3 = 2 , which is absurd, m=2 m = 2 and d3=4 d_3 = 4 . Hence, if we say d4=k d_4 = k :
n=12+22+42+k2=1+4+16+k2=21+k2 n = 1^2 + 2^2 + 4^2 + k^2 = 1 + 4 + 16 + k^2 = 21 + k^2
From kn k \mid n , we get k21 k \mid 21 , so k=7 k = 7 . However, this yields:
n=21+72=21+49=70 n = 21 + 7^2 = 21 + 49 = 70
This does not satisfy the conditions since 4 does not divide 70.

Thus, the only n n that satisfies the problem's conditions is 130 130 .

The final answer is 130 \boxed{130} .

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.