Examples of unary relation symbols?

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I Love Math

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I'm taking Foundations of Mathematics this semester (amongst three other math classes) and I'm having trouble understanding what a unary relation symbol would be. I've done a Google search (I found nothing) and a Wolfram search left me clueless.

I learn best by example and I'm not understanding how a relation symbol can be unary. It seems like an oxymoron.

How about 3-ary or 4-ary? Of course they exist or we wouldn't be talking about n-ary relation symbols, but does anyone here have any examples of them?

I'm most concerned about unary, though, since I'm going to need to use a unary relation symbol in one of our homework problems.

Thanks in advance!
 
I think unary operations would be the same thing as a unary function. For example, when I went to the instructor's office and asked what a unary function would be, he said that an exponent is a unary function since it performs an operation on a single number.

Unary functions are different than unary relations, though.

I was discussing this with one of the instructors at work (I work at a community college) and he suggested that a unary relation would be a relation with just a set of numbers.

I was thinking that I could list the natural numbers and the unary relation would be the question "Is this number prime?" then P(7), P(13), P(167) would all be true and P(4), P(28), P(144) would all be false. Or I'm wondering if I should list my set as the natural numbers and then list a separate set P of the primes. So then P of a prime is true and P of a composite number would be false.

I just wish I understood this stuff more.
 
Sounds practical, but what you described could also be considered a function... P: N -> {True, False}, where P(12)=False.
 
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