One day a gentleman entered the mason’s yard and offered him a certain price if he would supply him with a consecutive number of these cubical heaps which should contain altogether a number of blocks that could be laid out to form a square, but the buyer insisted on more than three heaps and declined to take the single block because it contained a flaw.  What was the smallest possible number of blocks of stone that the mason had to supply?

136.—­THE SULTAN’S ARMY.

A certain Sultan wished to send into battle an army that could be formed into two perfect squares in twelve different ways.  What is the smallest number of men of which that army could be composed?  To make it clear to the novice, I will explain that if there were 130 men, they could be formed into two squares in only two different ways—­81 and 49, or 121 and 9.  Of course, all the men must be used on every occasion.

137.—­A STUDY IN THRIFT.

Certain numbers are called triangular, because if they are taken to represent counters or coins they may be laid out on the table so as to form triangles.  The number 1 is always regarded as triangular, just as 1 is a square and a cube number.  Place one counter on the table—­that is, the first triangular number.  Now place two more counters beneath it, and you have a triangle of three counters; therefore 3 is triangular.  Next place a row of three more counters, and you have a triangle of six counters; therefore 6 is triangular.  We see that every row of counters that we add, containing just one more counter than the row above it, makes a larger triangle.

Now, half the sum of any number and its square is always a triangular number.  Thus half of 2 + 2 squared = 3; half of 3 + 3 squared = 6; half of 4 + 4 squared = 10; half of 5 + 5 squared= 15; and so on.  So if we want to form a triangle with 8 counters on each side we shall require half of 8 + 8 squared, or 36 counters.  This is a pretty little property of numbers.  Before going further, I will here say that if the reader refers to the “Stonemason’s Problem” (No. 135) he will remember that the sum of any number of consecutive cubes beginning with 1 is always a square, and these form the series 1 squared, 3 squared, 6 squared, 10 squared, etc.  It will now be understood when I say that one of the keys to the puzzle was the fact that these are always the squares of triangular numbers—­that is, the squares of 1, 3, 6, 10, 15, 21, 28, etc., any of which numbers we have seen will form a triangle.

Every whole number is either triangular, or the sum of two triangular numbers or the sum of three triangular numbers.  That is, if we take any number we choose we can always form one, two, or three triangles with them.  The number 1 will obviously, and uniquely, only form one triangle; some numbers will only form two triangles (as 2, 4, 11, etc.); some numbers will only form three triangles (as 5, 8, 14, etc.).  Then, again, some numbers will form both one and two triangles (as 6), others both one and three triangles (as 3 and 10), others both two and three triangles (as 7 and 9), while some numbers (like 21) will form one, two, or three triangles, as we desire.  Now for a little puzzle in triangular numbers.