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Number theory Difficulty 7.1 National olympiad, round 2 Find the answer

Find all integers a,b,c\,a,b,c\, with 1<a<b<c\,1<a<b<c\, such that (a1)(b1)(c1) (a-1)(b-1)(c-1) is a divisor of abc1.abc-1.

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

Solution

We are tasked with finding all integers a,b,c a, b, c with 1<a<b<c 1 < a < b < c such that

(a1)(b1)(c1) (a-1)(b-1)(c-1)

is a divisor of

abc1. abc - 1.

Let's first express abc1 abc - 1 in terms of potential divisors' expressions:

1. We want (a1)(b1)(c1)abc1(a-1)(b-1)(c-1) \mid abc - 1, meaning (a1)(b1)(c1)(a-1)(b-1)(c-1) divides abc1abc - 1.

Since (a1), (b1),(a-1), \ (b-1), and (c1)(c-1) are all positive integers greater than 1, we explore integer solutions systematically by substitution and testing constraints.

### Consider the case a=2a = 2:

1. bb and cc must satisfy abc10(mod(a1)(b1)(c1))abc - 1 \equiv 0 \pmod{(a-1)(b-1)(c-1)}. When a=2a = 2, the expression simplifies to:
bc10(mod(1)(b1)(c1)). bc - 1 \equiv 0 \pmod{(1)(b-1)(c-1)}.

2. Simplifying:
(b1)(c1)2bc1    bc1(mod(b1)(c1)). (b-1)(c-1) \mid 2bc - 1 \implies bc \equiv 1 \pmod{(b-1)(c-1)}.

Testing small integers bb and cc subject to 1<2<b<c1 < 2 < b < c:
- For b=4b = 4 and c=8c = 8:
- bc=32bc = 32 and (b1)(c1)=3×7=21(b-1)(c-1) = 3 \times 7 = 21.

Checking divisibility:
- 2bc1=641=63 2bc - 1 = 64 - 1 = 63,
- Since 630(mod21)63 \equiv 0 \pmod{21}, this implies (b,c)=(4,8)(b, c) = (4, 8) is valid for a=2a = 2.

Thus, (2,4,8)(2, 4, 8) is one solution.

### Consider the case a=3a = 3:

1. For a=3a = 3, we have:
(b1)(c1)3bc1. (b-1)(c-1) \mid 3bc - 1.

Exploring possible values of bb and cc given 1<3<b<c1 < 3 < b < c:
- For b=5b = 5 and c=15c = 15:
- bc=75bc = 75 and (b1)(c1)=4×14=56(b-1)(c-1) = 4 \times 14 = 56.

Calculating:
- 3bc1=2251=2243bc - 1 = 225 - 1 = 224,
- Since 2240(mod56)224 \equiv 0 \pmod{56}, this satisfies the condition.

Thus, (3,5,15)(3, 5, 15) is another solution.

### No Larger Values:
Looking for additional combinations of (a,b,c)(a, b, c) where 1<a<b<c1 < a < b < c, any further increase in a,b,ca, b, c leads to values of (a1)(b1)(c1)(a-1)(b-1)(c-1) that no longer satisfy the division condition when checked against new abc1abc - 1 values under these constraints, given a3a \leq 3.

Thus, the two valid triplet solutions found are:
(2,4,8)and(3,5,15) \boxed{(2, 4, 8)} \quad \text{and} \quad \boxed{(3, 5, 15)}

Both satisfy (a1)(b1)(c1)abc1(a-1)(b-1)(c-1) \mid abc - 1 and adhere to 1<a<b<c1 < a < b < c.

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