# Is there a specific theorem or name for this particular fact about primes? (Mod 6)

Is there a particular theorem or name defining the property/behavior of primes such that all primes (greater than 3) are congruent to 1 or 5 (mod 6)? I could have sworn I saw one years ago, but I haven't been able to find it again. I don't have a formal background in number theory, so that could be part of the reason, since I don't know the proper terms to search. I have been doing some work with Cuban primes. and I would like to know the name if there is one before speaking with a professor from a different country. The more we are on the same page the better.

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• Welcome to Maths SX! I don't think there's a special name. Simply, it' obvious that the other numbers, congruent to $0,2,3,4,$ can't be prime (except of course $2$ and $3$) since $6$ is divisible by $2$ and $3$. – Bernard May 14 at 21:49
• – J. W. Tanner May 14 at 21:55
• This observation was surely made shortly after primes have been "invented" ! – Peter May 14 at 21:56
• The term you're looking for may be "relatively prime", as in "$1$ and $5$ are the residues relatively prime to $6$". – Ethan Bolker May 14 at 22:04
• I am mostly looking for a term to make conversation/explanation more simple, like "and because we know theorem x to be true" or "and from theorem x" within the context of conversation. I already know how it works and how I am using them, but I am changing a one variable equation for a type of prime into a three variable problem. The visual presentation (being the most simple) is complicated enough, so I am going to try to find any ways I can to make the written/verbal part more simple and math friendly. – Mara May 14 at 22:45

• how does this fail ? $$1\cdot 1\equiv1 \bmod 6\\5\cdot 5\equiv 1\bmod 6$$ showing multiplicative inverse( last term). Furthermore: $$\gcd(1,6)=1\\gcd(5,6)=1$$ Showing coprimality ( fitting second term). Lastly, they fit all the group axioms with multiplication ( fit the first term) – Roddy MacPhee May 16 at 23:23