# Difficulties proving that a quadratic expression is composite

• Oct 19th 2012, 08:48 AM
matemauch
Difficulties proving that a quadratic expression is composite
I am trying to prove the following statement:
There is a quadratic f(n) = n2 + bn + c with positive coefficients b and c, such that f(n) is composite.
I am facing difficulties on how to approach this proof and right now I have no idea on how to start. Could you please give me a hint?

Thank you very much in advance.
• Oct 19th 2012, 08:59 AM
Plato
Re: Difficulties proving that a quadratic expression is composite
Quote:

Originally Posted by matemauch
I am trying to prove the following statement:
There is a quadratic f(n) = n2 + bn + c with positive coefficients a and b, such that f(n) is composite.

Where is any a?
• Oct 19th 2012, 12:14 PM
matemauch
Re: Difficulties proving that a quadratic expression is composite
I am sorry for the typo. It should be "...with positive integers b and c, such that..."

Thanks !
• Oct 20th 2012, 03:50 AM
matemauch
Re: Difficulties proving that a quadratic expression is composite
Quote:

Originally Posted by Plato
Where is any a?

Sorry for the typo. I think now it is correct.
• Oct 20th 2012, 04:38 AM
HallsofIvy
Re: Difficulties proving that a quadratic expression is composite
Take b= 3, c= 2. Then n^2+ 3n+ 2= (n+1)(n+2) is, for any n, the product of two integers and so composite.
• Oct 20th 2012, 07:14 AM
matemauch
Re: Difficulties proving that a quadratic expression is composite
Quote:

Originally Posted by HallsofIvy
Take b= 3, c= 2. Then n^2+ 3n+ 2= (n+1)(n+2) is, for any n, the product of two integers and so composite.

Thank you very much. I completely forgot about the factorization. Based on your suggestion I elaborated the next line of thought:
Let q and p positive integers.
Define b = x+y, and b is a positive integer
Define c = xy, and c is a positive integer
Then n^2 + bn + c = n^2 + (x+y)n + xy = (n+x)(n+y) is composite.

What do you think of it?
• Oct 26th 2012, 03:19 AM
Salahuddin559
Re: Difficulties proving that a quadratic expression is composite
What are you trying to prove here? I am lost in this, from what you are saying, x and y need not even be rational numbers, ok? Is that fine, having some irrational numbers in the factorization?

Salahuddin
Maths online
• Oct 26th 2012, 03:40 AM
matemauch
Re: Difficulties proving that a quadratic expression is composite
Quote:

Originally Posted by Salahuddin559
What are you trying to prove here? I am lost in this, from what you are saying, x and y need not even be rational numbers, ok? Is that fine, having some irrational numbers in the factorization?

Salahuddin
Maths online

Assume x and y be integers
• Oct 26th 2012, 08:27 PM
Salahuddin559
Re: Difficulties proving that a quadratic expression is composite
It is not always possible that x and y are integers. x and y are such that -x and -y are roots of this quadratic. And those are not always real numbers, even.

Salahuddin
Maths online