I was intrigued by a math puzzle posted to the SAS Discussion Forum (from New Scientist magazine). The problem is repeated below, but I have added numbers in brackets because I am going to comment on each clue:

[1] I have written down three different 5-digit perfect squares, which [2] between them use five different digits. [3] Each of the five digits is used a different number of times, [4] the five numbers of times being the same as the five digits of the perfect squares. [5] No digit is used its own number of times. [6] If you knew which digit I have used just once, you could deduce my three squares with certainty.

What are the three perfect squares?

I solved the problem by using the SAS/IML language. If you want to try solving the problem yourself without further hints, don't read any further!

### Deciphering the clues

I found the clues difficult to understand, so I'll give an example for each clue:

1. You are looking for three five-digit squares, such as 12321 = (111)2
2. There are five distinct digits among the 15 digits in the three numbers. For example, the numbers can't be (10000, 10201, 10404), because there are only four unique digits in these numbers: 0, 1, 2, and 4.
3. The frequencies of the five unique digits are distinct. Because there are 15 digits in the three numbers, we immediately know that the distribution of frequencies is 1, 2, 3, 4 and 5. So, for example, the numbers can't be (12321, 12544, 13225) because although there are five 2s and four 1s, no digit appears three times.
4. The five frequencies are the same as the five digits. Therefore, the five digits are 1, 2, 3, 4, and 5. For example, 57121 cannot be one of the numbers, because it contains a 7 among its digits.
5. The phrase "no digit is used its own number of times" baffled me. I finally figured out that it means that the digit 1 appears more than one time, the digit 2 appears either once or more than twice, the digit 3 does not appear exactly three times, and so forth.
6. At first I didn't realize that the last sentence is also a clue. The person who posted the puzzle said he found seven solutions, but the puzzle implies that there is a unique solution. The last sentence means that if you look at the frequency distribution of the digits among the seven potential solutions, only one of them is not a repeat of another. For example, the solution is not (12321, 244521, 355225) because the 3 digit appears once, but there is also another (potential) solution for which the 3 digit appears once.

When I wrote the SAS/IML program that solves the problem, I was pleased to discover that I had used three functions that are new to SAS/IML 9.3! Since the entire program is less than 25 lines long, that's a pretty good ratio of new functions to total lines. The three new functions I used are as follows:

• The ELEMENT function enables you to find which elements in one set are contained in another set. I use this function to get rid of all 5-digit perfect squares that contain the digits {6,7,8,9,0}.
• The ALLCOMB function generates all combinations of n elements taken k at a time. After I reduced the problem to a set of nine 5-digit numbers, I used the ALLCOMB function to look at all triplets of the candidate numbers.
• The TABULATE subroutine computes the frequency distribution of elements in a vector of matrix. I used this subroutine to apply rules 4 and 5.

You can use this post to ask questions or to clarify the directions. I will post my solution on Thursday. You can post your own solution as a comment to Thursday's post. DATA step and PROC SQL solutions are also welcome. Good luck!

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