( Elgin Johnston ) Legs of a square table each have length , where is a positive integer. For how many ordered 4-tuples of nonnegative integers can we cut a piece of length from the end of leg and still have a stable table?
(The table is stable if it can be placed so that all four of the leg ends touch the floor. Note that a cut leg of length 0 is permitted.)
Solution
The problem involves determining how many ways we can cut legs of a square table such that the table remains stable. Specifically, we aim to find the number of ordered 4-tuples where the lengths of the cuts are non-negative integers, and the legs of the table after cutting are stable, i.e., they can all touch the floor simultaneously.
### Stability Condition
For the table to be stable, the difference in lengths between the longest leg and the shortest leg should be at most the length of the shortest leg that remains. Thus, we need to ensure the difference between the longest and shortest cut is within the original length of the leg:
Simplifying this inequality, we have:
Which translates to:
Identifying as and as :
where , and .
This is equivalent to:
We redefine the variables to convert this into a combinatorial problem about distributing the reductions in leg lengths. Consider the space of all possible reductions in leg lengths and form a combinatorial problem:
### Combinatorial Translation
Since are non-negative integers and , if we let:
then we are essentially looking for the number of solutions in non-negative integers for:
where .
### Solving for
This problem is equivalent to distributing into 4 categories (legs), where each category can have zero units. This boils down to a classic stars and bars combinatorial problem, where the number of distributions of stars into 4 bars is:
To find the total count, we sum over all valid :
This simplifies, due to the combination identity of the sum:
This evaluates to:
Thus, the total number of valid configurations is given by: