Plethora of Pythagorean triples

True or False: Every odd number greater than 1 is the smallest member of a primitive Pythagorean triple.

Note: A primitive Pythagorean triple is a triple of positive integers ( a , b , c ) (a,b,c) such that a 2 + b 2 = c 2 a^2+b^2 = c^2 and gcd ( a , b , c ) = 1. \gcd(a,b,c) = 1.

False True Not enough info

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6 solutions

Brandon Monsen
Feb 23, 2017

if a < b < c a<b<c is a Pythagorean triple for odd a a , we know that a 2 = c 2 b 2 = ( b + c ) ( c b ) a^{2}=c^{2}-b^{2}=(b+c)(c-b)

If we let b , c b,c be consecutive integers we get that b + c = 2 b + 1 b+c=2b+1 and c b = 1 c-b=1 .

Therefore we want to confirm that for every odd a 3 a \geq 3 , there exists a positive integer integer b > a b >a such that a 2 = 2 b + 1 a^{2}=2b+1 .

Since a 2 a^{2} is odd and 2 b + 1 2b+1 can be any odd number, we simply need to make sure that b > a b > a in all cases. a 2 a^{2} grows faster that 2 b + 1 2b+1 for 3 a < b 3 \leq a<b , so if the smallest case works, they all work.

3 2 + 4 2 = 5 2 3^{2}+4^{2}=5^{2} works, so all off a 3 a \geq 3 are the smallest member of a Pythagorean triple.

Therefore it is True \boxed{\text{True}}

Although not directly related to this question, it's very interesting that the centre of the inscribed circle of a right angled triangle whose sides are a Pythagorean triple has integer coordinates relative to a vertex of the triangle!

John Foggitt - 1 year, 11 months ago

We can solve this. If a < b < c, let’s write a = 2n+1 for all n>0, b=2p and c=2p+1.

You can solve the a²+ b² + c² = 0. Replacing a, b and c gives you :

n² + n - p = 0

Δ = 4p + 1 > 0 so there are always 2 solutions.

The positive one is : n = -1/2 + sqrt(p + 1/4) Which can be rewritten as p = n² + n

We have our triplets for n>0

( 2n+1, 2n² + 2n, 2n² + 2n + 1)

And as b and c are consecutive, the gcd of the triplet is always 1.

So all odd numbers >1 are the smallest number of a Pythagorean’s triplet.

Nico Cale - 1 year, 7 months ago
Rob Matuschek
Sep 3, 2015

. Let a a be an odd positive integer greater than 1. Then a 2 + ( a 2 1 2 ) 2 = ( a 2 + 1 2 ) 2. a^{2}+(\frac{a^{2}-1}{2})^{2}= (\frac{a^{2}+1}{2})^{2.}

This stems from the "Pythagorean Triples Theorem."

I don't get your triple; the algebra does not work out.

Otto Bretscher - 5 years, 9 months ago

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Corrected, thank you!

Rob Matuschek - 5 years, 9 months ago

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Thanks. Those who previously attempted this question will be marked correct.

Brilliant Mathematics Staff - 5 years, 9 months ago

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@Brilliant Mathematics Thank you!

Rob Matuschek - 5 years, 9 months ago

8, 15, 17 make a triple, 8 = the smallest member is not odd and gcd (8,15,17)=1 ?! So...

Harout G. Vartanian - 4 years, 5 months ago

It's false Let a = 1 \Huge \text{Let}\, \, \, \, a=1

Isaac Buckley - 5 years, 9 months ago

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But the question stated for every odd number greater than 1. So a CANNOT equal 1.

mimked . - 5 years, 2 months ago

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They modified the question after I responded.

Isaac Buckley - 5 years, 2 months ago
Clay Creasy
Aug 5, 2017

If we use Euclid's formula for creating Pythagorean triples, but replace natural numbers ( m , n ) (m,n) where m > n m>n with ( m , m 1 ) (m,m-1) we get that the first term usually denoted by ( m 2 n 2 ) (m^2 - n^2) becomes the difference between two consecutive squares ( m 2 ( m 1 ) 2 ) (m^2 - (m-1)^2) which will simplify to the general term 2 m 1 2m-1 for all natural numbers m m greater than or equal to 2, thus creating a smallest term that represents the set of all the odd natural numbers.

For every odd number a a there exist two perfect squares x x and y y such that x + a = y x+a=y , with a < < x a<<x Let's consider the case where y = x + 1 y=x+1 The above statement is true in particular for ann odd number a a that is also a perfect square. The initial statement then in demonstrated because gcm(x,x+1) is always 1.

Affan Morshed
Apr 21, 2019

Let us first look at c=(b+1)^2, this is b^2+2b+1, 2b+1 describes any odd number for integer b, so this odd number could be the square of any odd number. Let us say this odd number, our odd number, is a=(2b+1)^0.5, we solve for b=((a^2)-1)/2, if we add 1 to get the sum in the Pythagorean triple we get c=((a^2)+1)/2. Now note that c=b+1 so these numbers have a gcd of 1, also do note that a^2 is a multiple of a, so (a^2)+1 and (a^2)-1, both of which have a difference of one, also have a gcd of 1 with a, and since the prime factorisation of the numbers is fundamentally different (no shared primes), and dividing by two would only take away primes (2) without adding primes, a shares no common factors with b or c above 1. We hence have an example where the gcd is one for every odd value above 1.

Piyush Raj
Aug 19, 2016

we can take the cases of --- 1st-3,4&5 2nd-5,12&13 and so on......

It is true if the smallest number is a prime number, not an odd number. What about Pythagorean triple 9, 12 15? gcd(9,12,15) = 3. But, for example, 13, 84, 85 ( easy to prove for the smallest prime number) there exist the primitive Pythagorean triple.

Larisa Altshuler - 4 years, 2 months ago

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