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Number theory Difficulty 8.3 Shortlist Find the answer

Find all positive integers nn for which all positive divisors of nn can be put into the cells of a rectangular table under the following constraints:
[list]
[*]each cell contains a distinct divisor;
[*]the sums of all rows are equal; and
[*]the sums of all columns are equal.
[/list]

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

Solution

Let us find all positive integers n n for which all positive divisors of n n can be placed into the cells of a rectangular table such that:

1. Each cell contains a distinct divisor of n n .
2. The sums of all rows in the table are equal.
3. The sums of all columns in the table are equal.

Consider a positive integer n n with divisors d1,d2,,dk d_1, d_2, \ldots, d_k . If these divisors can be arranged in a rectangular table with r r rows and c c columns such that each cell contains a distinct divisor, then the total number of divisors k k must satisfy k=r×c k = r \times c .

The sum of each row would be S S , and the sum of each column would be T T . This implies that the total sum of all divisors, denoted σ(n) \sigma(n) , must satisfy:
σ(n)=r×S=c×T. \sigma(n) = r \times S = c \times T.
Therefore, S=T S = T must be true for the sums of the rows and columns to be equal, and thus:
σ(n)=r×S=c×S. \sigma(n) = r \times S = c \times S.

Since rc=k rc = k and all divisors are used once, if we assume r1 r \neq 1 and c1 c \neq 1 , then the table's ability to balance row sums and column sums implies that divisors have to allow for equal distribution, which is a very restrictive condition.

For nontrivial values of n n with at least two distinct divisors, the structure implies more than one configurational constraint, leading us to check if there are simple cases where r=1 r = 1 , c=k c = k or vice versa, since σ(n) \sigma(n) is often a unique value with limited partitioning.

Through analysis or creating exhaustive cases, we find that the only integer n n which can ensure equality trivially when its divisors are structured in such a table is the simplest positive integer:

n=1. n = 1.

This is because n=1 n = 1 has exactly one divisor, itself, and thus neither multiple rows nor columns apply. The configuration automatically satisfies the constraints by default, as there is only a single 'cell' which naturally adheres to equal sums.

Thus, the positive integer n n for which all positive divisors can be arranged in such a way is:
1. \boxed{1}.

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