<s>[INST] Prove $(c^2-1)/4 \pm 1$ is not divisible by $2, 3, $ or $5$ if $c>3$ is an odd prime In this question asked the other day, RTn conjectured that, if $c>3$ is an odd prime, 
then $k=\dfrac{c^2\pm4-1}4=\dfrac{c^2-1}4\pm1$ is prime.  As the answers and comments there show, 
this conjecture is not true; some numbers of that form are multiples of $7$, $11$, $13$, etc.  
I suspect, however, that RTn was misled because $k$ cannot be divisible by $2$, $3$, or $5$.  
This question is how to prove this. 
I'm taking the option of answering my own question, but I'd appreciate feedback or other answers.
 [/INST] If $c>3$ is an odd prime, then $c$ is not divisible by $2, 3, $ or $5,$ 
unless $c=5, $ in which case the claim can be easily verified.
If $c$ is not divisible by $2,$ then $c^2-1=(c+1)(c-1)$ is the product of two consecutive 
even numbers so divisible by $8,$ so $\dfrac {c^2-1}4$ is even, so $\dfrac{c^2-1}4\pm1$ is not divisible by $2$.
 If $c$ is not divisible by $3$, then $c+1$ or $c-1$ is, so $c^2-1$ is, so $\dfrac{c^2-1}4$ is (since $4\equiv1\mod3),$ 
so $\dfrac{c^2-1}4\pm1$ is not.  
Finally, if $c$ is not divisible by $5,$ 
 then $c^2\equiv 1$ or $4\mod5$, so $c^2-1\equiv 0$ or $3\mod 5$; 
since $4\equiv-1\mod 5,$ that means $\dfrac{c^2-1 }4\equiv0 $ or $2\mod 5$, so $\dfrac{c^2-1}4\pm1\not\equiv0\mod5.$
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