Maths Olympiad Prep

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Combinatorics Difficulty 6.2 National olympiad Find the answer

Let A={1,2,,n}A=\{1,2,\ldots,n\}. Find the number of unordered triples (X,Y,Z)(X,Y,Z) that satisfy XYZ=AX\bigcup Y \bigcup Z=A

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

Solution

To find the number of unordered triples (X,Y,Z)(X, Y, Z) that satisfy XYZ=AX \cup Y \cup Z = A where A={1,2,,n}A = \{1, 2, \ldots, n\}, we need to consider the following steps:

1. **Counting the ways to assign each element to subsets XX, YY, and ZZ:**
Each element in AA can be in any of the subsets XX, YY, or ZZ, or in any combination of these subsets. There are 23=82^3 = 8 possible ways to assign each element to the subsets XX, YY, and ZZ (since each element can either be in or out of each subset).

2. Excluding the empty assignment:
Out of these 8 possibilities, one possibility is that the element is not in any of the subsets XX, YY, or ZZ. This is not allowed since XYZ=AX \cup Y \cup Z = A must cover all elements of AA. Therefore, we have 81=78 - 1 = 7 valid ways to assign each element to the subsets.

3. Calculating the total number of assignments:
Since each of the nn elements in AA can be assigned to the subsets in 7 different ways, the total number of ways to assign all elements is 7n7^n.

4. Considering unordered triples:
The problem asks for unordered triples (X,Y,Z)(X, Y, Z). However, since the assignment of elements to subsets is inherently ordered, we do not need to further adjust for ordering.

Thus, the total number of unordered triples (X,Y,Z)(X, Y, Z) that satisfy XYZ=AX \cup Y \cup Z = A is 7n7^n.

The final answer is 7n\boxed{7^n}

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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.