Why does (p*q+2)-(p+q) always give a prime number?

In summary, the conversation discusses a question about a formula involving prime numbers and how it always gives a prime number as a result. However, it is pointed out that this is not always the case and counter examples are provided. The conversation also touches on the importance of trying multiple examples before generalizing.
  • #1
John Harris
19
1

Homework Statement


Why does (p*q+2)-(p+q) always give a prime number when p and q are prime? Is there a similar formula that would prove this

Homework Equations


That's what I'm looking for. It might have something to do with Eulers formula

The Attempt at a Solution


I tried to find online a formula that would justify this, but was unable to find anything.
 
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  • #2
John Harris said:
Why does (p*q+2)-(p+q) always give a prime number when p and q are prime?
It doesn't.

The first example I picked was a counter example:
(17*47+2)-(17+47)=737=11*67
 
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  • #3
Nathanael said:
It doesn't.

The first example I picked was a counter example:
(17*47+2)-(17+47)=737=11*67
Oh you're right. I should have tried more examples. Thank you
 
  • #4
John Harris said:
Oh you're right. I should have tried more examples. Thank you
You couldn't have tried many. It fails whenever p and q differ by 2.
 
  • #5
haruspex said:
You couldn't have tried many. It fails whenever p and q differ by 2.
I tried 7 and 13
 
  • #6
John Harris said:
I tried 7 and 13
Only that pair?! Try 3 and 5, 5 and 7, 11 and 13,...
 
  • #7
John Harris said:
I tried 7 and 13
And you think one example is enough to generalize from ? Probably not a great idea.
 
  • #8
Gez no I tried 3 examples 3,7 1,3 7,13, and it would have made sense with the problem I'm doing.
 
  • #9
John Harris said:
Gez no I tried 3 examples 3,7 1,3 7,13, and it would have made sense with the problem I'm doing.
Well can you see how your statement "I tried 7 and 13" sounds a LOT like "I tried one combination" ? Glad to hear you already realize that just one is not a good idea.
 
  • #10
John Harris said:
Gez no I tried 3 examples 3,7 1,3 7,13, and it would have made sense with the problem I'm doing.
Your second example isn't valid because 1 isn't a prime number.
 

Related to Why does (p*q+2)-(p+q) always give a prime number?

1) Why is it important to study the equation (p*q+2)-(p+q) and its relation to prime numbers?

Studying this equation can provide insight into the properties and behavior of prime numbers, which are fundamental building blocks in mathematics and have important applications in computer science and cryptography.

2) Is there a proof that (p*q+2)-(p+q) always results in a prime number?

No, there is currently no proof that this equation always results in a prime number. It is a conjecture that has yet to be proven or disproven.

3) Are there any known counterexamples to this equation resulting in a prime number?

No, there are no known counterexamples. However, the fact that it has not been proven adds to the intrigue and mystery of this equation.

4) Can this equation be simplified or rewritten in a different form?

Yes, this equation can be rewritten as (p-1)(q-1)+1, which is known as the Euler's totient function. This function has been extensively studied and is closely related to prime numbers.

5) Are there any real-world applications for this equation and its relationship to prime numbers?

Yes, this equation plays a crucial role in the RSA algorithm, which is widely used for secure communication and data encryption. Understanding the properties of this equation can help in improving the security of RSA and other cryptographic systems.

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