Let . Prove that can be written as a product of nine different integers all between 16 and 30 inclusive.
Solution
There is a unique such representation:
All solutions start by factorising into primes:
This can be obtained manually or by Legendre's formula for the highest power of that divides viz.
Systematic Solution
As there is only one multiple of and one multiple of between and inclusive, two of the factors in the product must be and .
There are two multiples of , and no multiples of between and , so both and must feature in the product.
There are three multiples of between and , one of which is . So the multiples of in the product must be either and , or and .
Taking out the factors already considered:
To finish the problem we seek four further factors between and (inclusive) excluding those already used.
Of the remaining numbers between and , only are multiples of . If we do not choose , then we can reach at most in the product, which is short of the we need. Therefore, must be in the list of factors. Dividing through by , we are left to find three factors of:
The feasible factors must only have prime factors of , and/or . Excluding factors already used, we have the following list:
| Include | Factor | Prime factorisation |
|---|---|---|
| YES | 16 | |
| YES | 18 | |
| YES | 20 | |
| NO | 24 | |
| NO | 30 | |
| Product | 5760 |
Start with and starting from , divide repetitively by the largest remaining number that is a factor.
Taking the largest possible number at each stage
This has nearly worked, but the final factor of is problematic.
So we tweak a bit, first replacing and by and
This is still not a solution, but our smallest factor has increased a bit. As a final step, replace and by and which gives the solution stated.