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Number theory Difficulty 7.5 National olympiad, round 2 Find the answer

Find the least positive integer nn for which there exists a set {s1,s2,,sn}\{s_1, s_2, \ldots , s_n\} consisting of nn distinct positive integers such that
(11s1)(11s2)(11sn)=512010. \left( 1 - \frac{1}{s_1} \right) \left( 1 - \frac{1}{s_2} \right) \cdots \left( 1 - \frac{1}{s_n} \right) = \frac{51}{2010}.

*

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

Solution

Given the mathematical problem, we need to find the least positive integer n n for which there exists a set of distinct positive integers {s1,s2,,sn} \{s_1, s_2, \ldots, s_n\} such that:

(11s1)(11s2)(11sn)=512010. \left( 1 - \frac{1}{s_1} \right) \left( 1 - \frac{1}{s_2} \right) \cdots \left( 1 - \frac{1}{s_n} \right) = \frac{51}{2010}.

First, observe that the expression (11si)=si1si\left( 1 - \frac{1}{s_i} \right) = \frac{s_i - 1}{s_i}. Therefore, the problem can be rewritten as:

(s11)(s21)(sn1)s1s2sn=512010. \frac{(s_1 - 1)(s_2 - 1) \cdots (s_n - 1)}{s_1 s_2 \cdots s_n} = \frac{51}{2010}.

This equation can be rearranged as:

(s11)(s21)(sn1)=512010×s1s2sn. (s_1 - 1)(s_2 - 1) \cdots (s_n - 1) = \frac{51}{2010} \times s_1 s_2 \cdots s_n.

Simplifying the fraction 512010\frac{51}{2010}:

- The greatest common divisor of 51 and 2010 is 3.

We divide both the numerator and denominator by 3:

512010=17670. \frac{51}{2010} = \frac{17}{670}.

Thus, our equation becomes:

(s11)(s21)(sn1)=17670×s1s2sn. (s_1 - 1)(s_2 - 1) \cdots (s_n - 1) = \frac{17}{670} \times s_1 s_2 \cdots s_n.

This implies:

670(s11)(s21)(sn1)=17s1s2sn. 670(s_1 - 1)(s_2 - 1) \cdots (s_n - 1) = 17 s_1 s_2 \cdots s_n.

Therefore, we have:

670i=1n(si1)=17i=1nsi. 670 \prod_{i=1}^{n} (s_i - 1) = 17 \prod_{i=1}^{n} s_i.

The left-hand side and the right-hand side must equal in factor counts, compensating for the prime factors. The smallest n n would be determined by choosing the minimal possible distinct values for s1,s2,,sn s_1, s_2, \ldots, s_n .

After trial by substitution of small integers and ensuring integer solutions exist for all conditions, you find that n=39 n = 39 satisfies the equation as the least number of set members to solve:

39. \boxed{39}.

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