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Baltic Way 2020 · Problem 19

Number Theory

Denote by d(n)d(n) the number of positive divisors of a positive integer nn. Prove that there are infinitely many positive integers nn such that ⌊3 d(n)⌋\lfloor \sqrt{3}\,d(n)\rfloor divides nn.

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Topics

Divisibility and factorization · Arithmetic functions

Solutions

Solution

Note that ⌊3⋅8⌋=13\lfloor \sqrt{3} \cdot 8 \rfloor = 13. Therefore all numbers with 88 divisors that are divisible by 1313 satisfy the condition. There are infinitely many of those, for example, all numbers in the form 13p313p^3, where pp is a prime different from 1313.

Contest context

Results from Baltic Way 2020

10 teams

Mean score
2.1 / 5
Scores of 4 or 5
3 / 10
Estonia
1 / 5

Score distribution

03
13
20
31
40
53
All team scores
TeamScore
Germany5 / 5
Norway5 / 5
Poland1 / 5
Finland5 / 5
Latvia0 / 5
Estonia1 / 5
Denmark0 / 5
Sweden1 / 5
Lithuania3 / 5
Iceland0 / 5