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I have arrived to this conjecture in my work, I am not sure that is true or false. So I would appreciate if someone give a counterexample or prove it.

My question: Let n be a non-prime such that n-1 be a prime. Is it true that there exist a prime p such that n-p is a prime as well?

edited Aug 13 at 14:02

Benjamin Steinberg
10.7k●1●14●49

asked Aug 13 at 7:05

Tina
29●2

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This is exactly the special case of Goldbach's Conjecture for even numbers that are 1 more than an odd prime. – quasi-coherent Aug 13 at 7:27

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Could you give some more details about the work where this question arose? – Yemon Choi Aug 13 at 7:58

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In Goldbach's day, the number 1 counted as a prime, hence the clean conjecture that "every even number is the sum of two (old-fashioned) primes." The modern version, namely that every even number is the sum of two (modern) primes, is stronger exactly by the gap which is the OP's conjecture, hence some natural interest. – David Feldman Aug 13 at 9:33

retagged as open problem. – Benjamin Steinberg Aug 13 at 14:03

Deal all thank you so much for you answers. Actually I have arrived to this question in group theory and I have never thought that is an old open problem. I would appreciate, if you let me know any approach concerns this special case. – Tina Aug 15 at 10:22

S. Carnahan · Accepted Answer · 2012-08-13 09:06:25Z UTC

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The conjecture that any even number (greater than 4) is the sum of two odd primes is well-known to be open. You are asking about a special case that is also open.

answered Aug 13 at 9:06

S. Carnahan♦

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In view of David Feldman's comment, I add that the way this (and these types of question) are approached and seen nowadays this special case seems essentially as hard as the full conjecture. – quid Aug 13 at 13:07

A question concerns prime numbers

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