What Is the Flaw in This Proof of the Greatest Common Divisor?

In summary, the conversation discusses the proof of d^2 being the greatest common divisor of a^2 and b^2, with one person realizing that simply stating d^2 as a common divisor does not necessarily mean it is the greatest common divisor.
  • #1
kathrynag
598
0

Homework Statement


Suppsoe a, b[tex]\in[/tex]natural numbers, and d=GCD(a,b). Then d^2=GCD(a^2,b^2). I need to find where the proof goes wrong.


Homework Equations





The Attempt at a Solution


By hypothesis, we have that d divides a and d divides b, so there are integres s and t with a=ds and b=dt. Then a^2=d^2s^2 and so d^2 divides a^2. Similarily d^2 divides b^2. Thus d^2 is a common divisor of a^2 and b^2, as desired.
 
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  • #2


No, not "as desired". What you "desire" is the greatest common divisor. Saying that d^2 is a common divisor does not mean it is the GREATEST common divisor.
 
  • #3


HallsofIvy said:
No, not "as desired". What you "desire" is the greatest common divisor. Saying that d^2 is a common divisor does not mean it is the GREATEST common divisor.

Ok, that made a lot more sense now that I see that!
 

What is a greatest common divisor (GCD)?

A greatest common divisor (GCD) is the largest positive integer that divides evenly into two or more given integers. It is also known as the greatest common factor (GCF) or highest common factor (HCF).

How do you find the GCD of two numbers?

The most common method to find the GCD of two numbers is by using the Euclidean algorithm. This involves repeatedly dividing the larger number by the smaller number until the remainder is 0. The last non-zero remainder is the GCD.

What is the relationship between GCD and prime numbers?

Prime numbers are integers that are only divisible by 1 and themselves. The GCD of two prime numbers is always 1 because they do not have any common factors other than 1.

How is GCD used in mathematical operations?

GCD is used in simplifying fractions, finding the least common multiple (LCM), and solving linear Diophantine equations. It is also used in cryptography and computer algorithms.

Can the GCD of two numbers be negative?

No, the GCD is always a positive integer by definition. However, if one or both of the numbers are negative, the GCD will remain the same as if they were positive.

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