Can two consecutive odd primes sum to a product of three integers?

  • Context: Graduate 
  • Thread starter Thread starter Andromache
  • Start date Start date
  • Tags Tags
    Prime Proof
Click For Summary

Discussion Overview

The discussion revolves around whether the sum of any two consecutive odd prime numbers can be expressed as the product of three integers, all greater than 1. Participants explore various mathematical properties and implications of this conjecture, including the nature of prime numbers and their sums.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asserts that the sum of two consecutive odd primes can be expressed as a product of three integers greater than 1.
  • Another participant challenges this by providing a counterexample involving the primes 7 and 11, suggesting the conjecture is false.
  • A subsequent reply clarifies that the original question pertains to the sum, not the product, indicating a misunderstanding.
  • One participant proposes that if p and q are consecutive odd primes, then (p+q)/2 cannot be prime, as there are no primes between consecutive primes.
  • Another participant elaborates that since p+q is even, it can be factored into 2 and another integer, leading to the conclusion that the sum can be expressed as a product of three integers greater than 1 in both even and odd cases.
  • A later reply supports the argument that (p+q)/2 is composite, reinforcing the idea that the sum can be expressed as a product of integers greater than 1.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the conjecture, with some providing counterexamples and others defending the possibility of expressing the sum as a product of three integers. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Some arguments depend on the definitions of prime numbers and the properties of sums of consecutive primes. The discussion includes assumptions about the nature of even and odd integers and their factors.

Andromache
Messages
7
Reaction score
0
Prove that the sum of any two consecutive odd prime numbers can always be written as the product of three integers, all greater than 1.

I'm sure this is simpler than it looks. Any help?
 
Physics news on Phys.org
7x11 =77, which is composed only of 7 and 11, so the conjecture is false.
 
robert Ihnot said:
7x11 =77, which is composed only of 7 and 11, so the conjecture is false.

I think you misread the question. It's the sum not product.
 
Suppose p and q are consecutive odd primes. Can (p+q)/2 be prime?
 
if p and q are consecutive there isn't any prime number that is "between" the consecutive prime numbers p and q, so (p+q)/2 isn't a prime number.
 
p+q is an even number, so we have that there are two factors: 2 and a (even or odd) number. (p+q)/2 could be even or odd. If it's even we have that the sum of two consecutive prime numbers can be written as (2)x(2)x(number). If it's odd we have that it isn't a prime numbers because there isn't any prime number between two CONSECUTIVE prime numbers, so we can write the odd number (p+q)/2 as (odd number)x(another odd number) and the sum of the 2 consecutive prime numbers as (2)x(odd number)x(another odd number). In both cases we demonstrate that the sum of two consecutive prime numbers can always be written as the product of three integers, all greater than 1.
 
I think it's more straightforward starting from CRGreathouse's suggestion directly.

(p+q)/2 is not prime because, as Andrea said, it's between p and q, which are consecutive primes.

Now, if (p+q)/2 is composite, there are two integers a,b > 1 such that ab = (p+q)/2, and therefore 2ab = p + q. QED.
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 10 ·
Replies
10
Views
3K
Replies
9
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
6
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 44 ·
2
Replies
44
Views
4K
Replies
9
Views
2K