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As of October, 2016, Embarcadero is offering a free release of Delphi (Delphi 10.1 Berlin Starter Edition ). There are a few restrictions, but it is a welcome step toward making more programmers aware of the joys of Delphi. They do say "Offer may be withdrawn at any time", so don't delay if you want to check it out. Please use the feedback link to let me know if the link stops working.

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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.)

Contact

Feedback: Send an e-mail with your comments about this program (or anything else).

Problem Description

Put 3 marbles in a triangle and place a 4th marble on top. You have just built a triangular based pyramid, also called a tetrahedron. If you started with 6 marbles and placed a second layer of 3 and then one on top, you have another tetrahedron with 3 layers and 10 marbles. The two layer tetrahedron contains 4 marbles, a perfect square(4=2²). Can you find the next tetrahedron whose total number of marbles is a perfect square?

Background & Techniques

Only about 10 lines of code in this one. Starting with a one marble pyramid, we'll just generate the total for each pyramid from the previous one. And stop when the square root of the total is an integer.

I put an upper limit of 1000 marbles per side, just in case there weren't any small solutions. We don't want the program to loop forever. Notice that each layer contains the number of marbles in the previous layer plus the layer number additional. And the total is just the previous total plus the number in this layer.

Tetrahedron #	Marbles in bottom layer	Total marbles
1	1	1
2	3	4
3	6	10
4	10	20
5	15	35

Running/Exploring the Program

	Browse source extract
	Download source
	Download executable

Suggestions for Further Explorations

It turns out that there are names for each of the numbers we worked with in this program. The number of marbles in each layer are triangular numbers. And the total number of marbles in each pyramid is a tetrahedral number. The solution found here is the largest one. And, as you might suspect, there are algorithms to compute the millionth tetrahedral number without computing the first 999,999. For more information, check this excellent site: http://mathworld.wolfram.com/TetrahedralNumber.html

Triangular numbers have other interesting properties. For example

	The sum of any two consecutive triangular numbers is the square of an integer!
	The square of the Nth triangular number minus the square of its predecessor is Ncubed!
	The Nth triangular number is also the number of ways we can select 2 objects from N+1, ignoring order (these are called combinations). For example, the 3rd triangular number from the table above is 6. and there are exactly 6 combinations for 2 of 4 objects. See Permutes2 program for more discussion of combinations.

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