Problem:
Determine the number of ordered triples of nonnegative integers such that each of the numbers , , is less than and the number is a divisor of .
Problem:
Determine the number of ordered triples of nonnegative integers such that each of the numbers , , is less than and the number is a divisor of .
Pick one
Solution:
The answer is . Let us start by observing that the following algebraic identity holds
Setting , , , the condition is equivalent to ; in particular, we want to look for the triples such that .
If we assume , simplifying a we obtain that . If it were , on the left we would have an odd number , while on the right a number whose odd part is 3, which is impossible. Hence holds and we must find the solutions of . At this point we split into four cases
- If , is always satisfied, so we have all triples of the form with (since ), which are 14.
- If , we can rewrite the condition as , which holds for and from which we obtain the triples and their permutations; hence this case has in total solutions (since we also need )
- If , the condition becomes , which holds for . We thus obtain the triples and their permutations, which are valid up to , hence solutions in this case as well.
- If , letting we have . Since , the number on the left is odd and , so there are no solutions.
The total number of ordered triples is therefore .