Prime Pair Sums

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Mensa Daily Puzzlers

For over 15 years Mensa Page-A-Day calendars have provided several puzzles a year for my programming pleasure.  Coding "solvers" is most fun, but many programs also allow user solving, convenient for "fill in the blanks" type.  Below are Amazon  links to the two most recent years.

Mensa 365 Puzzlers  Calendar 2017

Mensa 365 Puzzlers Calendar 2018

(Hint: If you can wait, current year calendars are usually on sale in January.)


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Problem Description

.The object of this puzzle is to arrange the integers 1 through 16 into a 4x4 grid so that the sum of any two cells horizontally or vertically is a prime number (no divisors except 1 and itself).

Background & Techniques

Here is one of the  2992 solutions which can be generated by the program.  Since the only even prime is 2 and the sum of adjacent cells will always be 3 or greater, all of the prime sums must be odd. And, since the sum of two even or two odd numbers is always even, it is clear that each number must have neighbors of the opposite parity.  If a row contains odd-even-odd-even numbers, the row above or below must contain even-odd-even-odd numbers. The effect is similar to a checkerboard with number of one parity on the black squares and numbers of the other parity on the red squares.

No user play is included in the program, so you're on your own if you want to try discovering a solution without help.  The puzzle was adapted from "The Master Book of Mathematical Recreations", Fred Schuh, Dover Publications.  The book has several pages describing techniques for finding solutions manually.   The book is out of print, but you can use the Amazon link in the sidebar at left to find used copies. 

The program takes advantage of the "checkerboard" arrangement of odds and evens  by permuting the 8 odd numbers into the "odd" positions and for each of those, permuting the 8 even numbers into the "even" positions and then checking for prime pair sums.  For a second "Solve" button, the odd-even roles are reversed. Note that if the effects of rotating and mirroring are considered, the number of unique solutions is reduced by a factor of 8.

Non-programmers are welcome to read on, but may want to skip to the bottom of this page to download executable version of the program.

Notes for Programmers

Upon first reading the problem description, it seemed to be quite trivial, just generate all permutations of the integers 1 to 16 and check the sums appropriately.   For the 12 horizontal pair sums,( positions 1, 2, 3, 5, 6, 7, 9, 10, 11, 13, 14, 15), sum  with the next number and check to see if the sum is prime.  For positions 1 through 12, check the vertical pairs by summing the number in that position with the one 4 positions later (position 1 + position 5,  pos. 2 + pos  6, etc.). 

The problem is that there would be 20 trillion permutations to check (2x1013).   A little further thought made it clear that there is no need to check all permutations.  As explained above, odd and even numbers must be adjacent, so we can permute the odds and evens separately and insert them in an array appropriately to perform the sum check.  There are "only" 40,000  permutations of 8 objects, so we can get by with checking (4 x 104) 2  = 1.6 x 109 permutations,   reducing the problem size by a factor of 10,000 and allowing solutions to be found in a minute or less on a modern computer. 

Once an array is ready to be checked, the fastest way turned out to be testing that a sum was not one of the five non-prime odd numbers between  the smallest sum (1+2=3) and the largest (15+16=31).   The non-primes  in this range are 9, 15, 21, 25, and 27.  Finding any of these values among the 24 sums to check allow us to stop checking that array immediately.

The MathsLib unit is included in the downloadable source zip file.  It contains the NextPermute function used to  generate the permutations.   The MathsLib version included in DFFLibV13 has NextPermute code in the Implementation section but omitted from the Interface section.  That will be corrected in the next library release. 

By the way, in case you haven't caught on to the trick to converting a one dimensional array into a two dimensional grid, here's a small table converting 8 array entries into a 4 column x 2 row grid using mod (modulo) and div (integer division) operations.. Note that all numbering starts a 0. This is the default for most table indexing because it simplifies the math.  If you are given or prefer to number from 1 as most humans do, then it's  a matter of subtracting 1 from the position before doing the "mod" or "div" operation and adding 1 back to the result if necessary.

1-Dimensional position, P 0 1 2 3 4 5 6 7
2-D Column, C = P mod 4 0 1 2 3 0 1 2 3
2-D Row, R = P div 4 0 0 0 0 1 1 1 1


Running/Exploring the Program 

bullet Download source
bullet Download  executable

Suggestions for Further Explorations

bullet The NextPermute function used to generate permutations the does not provide an easy way to "prune" the search by skipping ahead when appropriate.  For example, when the odd permutations start with 5, it would be nice to skip generating the even permutations starting  with 4 or 10  since they will combine to for sums 9 or 15, not prime.
bullet Users could be given help in finding a solution if an option were provided to display only the odd (or even) numbers of a solution.   
bullet Are there "wraparound" solutions? (Last column paired with first column and last row paired with first row.)   No. ( I couldn't resist making a version to check.)


Original Date: May 18, 2009 

Modified: May 15, 2018


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