Sum of Distinct Primes: Fact or Fiction?

  • Context: Undergrad 
  • Thread starter Thread starter chhitiz
  • Start date Start date
  • Tags Tags
    Primes Sum
Click For Summary

Discussion Overview

The discussion revolves around the proposition that every composite number, with the exceptions of 4 and 6, can be expressed as the sum of distinct prime numbers. Participants explore the validity of this claim, potential proofs, and its relation to established conjectures in number theory.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant observes that every composite number, except for 4 and 6, can be expressed as a sum of distinct prime numbers, providing examples like 200 and 100.
  • Another participant suggests that the claim could potentially be proven using mathematical induction, outlining a method involving base cases and inductive steps.
  • A third participant mentions that Bertrand's postulate could be used to prove the claim.
  • It is proposed that the number of distinct primes in such sums does not exceed log2(N), where N is the composite number being decomposed.
  • One participant draws a parallel to Goldbach's conjecture, noting its historical significance and the lack of a proof, suggesting that the current claim may also be difficult to prove.
  • Another participant argues that the current proposition is 'easier' than Goldbach's conjecture, as it allows for any number of distinct primes rather than being limited to two.
  • There is a suggestion that Bertrand's postulate may be sufficient for proving the original claim.

Areas of Agreement / Disagreement

Participants express differing views on the validity and provability of the claim. While some propose methods for proof and reference established conjectures, there is no consensus on whether the original proposition is true or can be definitively proven.

Contextual Notes

Some assumptions and definitions are not fully explored, such as the implications of Bertrand's postulate and the specific conditions under which the claim holds. The discussion also highlights the complexity of proving statements related to prime numbers and their sums.

chhitiz
Messages
223
Reaction score
0
i observed that every composite no. with exception of 4 and 6 can be expressed as sum of distinct prime no.s. eg: 200=103+97 100=53+47 25=13+7+5
is this true? is there any theorem stating as such?
 
Physics news on Phys.org
Interesting question!

It seems true. Maybe you could prove it using induction?

The base cases could be integers from 1 to 20, say, by just showing the sums.

Then you could assume that it works for all composite numbers up to a composite number k.

Then does it work for the next composite number? Well, look at the largest prime number less than it. If composite - prime is not 1 or 4 or 6, then the sum can certainly be expressed as prime + (sum for the difference composite - prime, which can definitely be done... if the difference is prime, just take the prime, otherwise, the inductive hypothesis covers it). If the difference is 1 or 4 or 6, try the next prime number below the one you just tried. If this isn't 4 or 6, do the same thing. If it is, try the next lower prime number. And etc.

21 = composite, 19 = prime, diff = 2, done with sum = 19 + 2.

22 = composite, 19 = prime, diff = 3, done with sum = 19 + 3.

24 = composite, 23 = prime, diff = 1. Try 19 = prime, then diff = 5 and done with 19 + 5.

25 = composite, 23 = prime, diff = 2, done with 23 + 2.

26 = composite, 23 = prime, diff = 3, done with 23 + 3.

27 = composite, 23 = prime, diff = 4, try 19 = prime, diff = 9, done with 19 + psum(9) = 19 + 7 + 2.

etc.

This is just a sketch, but I think it should work. Does anybody have any comments?
 
You can prove this with Bertrand's postulate.
 
Yes, indeed.
It also follows from Bertrand's postulate that the number of such distinct primes does not exceed log2(N), where N is the original number to be decomposed.
Am I correct?
 
This is almost identical to 'Goldbach's conjecture' which is more than two and a half centuries old, famous, and has never been proved, so you are unlikely to.
 
This one is 'easier' in the sense that there is no limit on the number of primes that make the sum, just that they are distinct, whilw Goldback's conjecture restricts to two primes. It seems to me that Bertrand's postulate is sufficient for the proof.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
4K
Replies
3
Views
2K
  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 2 ·
Replies
2
Views
6K
  • · Replies 5 ·
Replies
5
Views
5K
  • · Replies 80 ·
3
Replies
80
Views
10K
  • · Replies 93 ·
4
Replies
93
Views
17K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 3 ·
Replies
3
Views
4K