Question: How to check if a property applies to all coprimes of a number?

Hi, I'm rather new to Maple but I have to find a composite number n which has the property that all her coprimes a fulfill the expression "a^(n-1) - 1 mod n = 0". I already found out that such numbers are called Carmichael numbers. But of course I need to use Maple and the information I was given by our professor. My problem is that I have no idea how to check if "a^(n-1) - 1 mod n = 0" is true for all coprimes of a given n. How do I? ;) btw. I don't know if I used the correct vocabulary here, so please forgive me if I expressed something wrong.
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