Pythagorean Triples - Advanced

(You may like to read Pythagoras' Theorem
and Introduction to Pythagorean Triples first)

A "Pythagorean Triple" is a set of positive integers a, b and c that fits the rule:

a² + b² = c²

Triangles

And when we make a triangle with sides a, b and c it will be a right angled triangle (see Pythagoras' Theorem for more details):

Note:

• c is the longest side of the triangle, called the "hypotenuse"
• a and b are the other two sides

Pythagorean Triples

A famous example of a Pythagorean Triples:

The 3,4,5 Triangle

32 + 42 = 52

9 + 16 = 25

Two more examples:

5, 12, 13		9, 40, 41
52 + 122 = 132		92 + 402 = 412
25 + 144 = 169		(try it yourself)

Endless

The set of Pythagorean Triples is endless.

We can prove this with the help of the first Pythagorean Triple (3, 4, 5):

Primitive Triples

In the case above (3,4,5) is a primitive triple,

But all its multiples, such as (6,8,10) etc, are not.

Primitive triples have this property: a, b and c share no common factors.

Euclid's Proof of Infinitely Many Pythagorean Triples

But Euclid used a different reasoning to prove the set of Pythagorean Triples is unending.

The proof was based on the fact that the difference of the squares of any two consecutive (one after the other) whole numbers is always an odd number.

Can you see how subtracting squares make odd numbers in this picture?

See Squares and Odd Numbers, or have a look at this table as an example:

And there are an infinite number of odd numbers. Since the perfect squares form a subset of the odd numbers, and a fraction of infinity is also infinity, it follows that there must also be an infinite number of odd squares. So there are an infinite number of Pythagorean Triples.

Properties

An interesting fact: a Pythagorean Triple always consists of:

• all even numbers, or
• two odd numbers and an even number.

A Pythagorean Triple can never be made of all odd numbers or two even numbers and one odd number. This is true because:

• The square of an odd number is an odd number and the square of an even number is an even number.
• The sum of two even numbers is an even number and the sum of an odd number and an even number is in odd number.

So, when both a and b are even, c is even too. Similarly when one of a and b is odd and the other is even, c has to be odd!

Constructing Pythagorean Triples

It is easy to construct sets of Pythagorean Triples.

When m and n are any two positive integers (m > n):

• a = m² − n²
• b = 2mn
• c = m² + n²

Then a, b and c form a Pythagorean Triple. This is known as "Euclid's formula".

This method creates all primitive triples, but we may need to swap a and b to see:

It also creates some non-primitive triples (that are multiples of primitive triples):

List of the First Few

Here is a list of all primitive Pythagorean Triples for a, b, and c less than 1000.

The list has only primitive triples, so (3,4,5) is there, but (6,8,10) etc are not