What do you call a set of friendly numbers whose squared values are less than or equal to zero?
Imaginary Friends!
(Mods, if necro-posting is actually kosher around here then please combine with one of the other ones. I’m simply accustomed to such being frowned upon.)
I have to confess I don’t quite know what you mean, being ignorant of conventional netiquette. I figure it’s a necro-post if A) it’s, well, dead, and B) it smells bad. So far you’ve taken care of B. 
“Necro-posting” is adding a new post to a very old
thread (thus “resurrecting” by bringing it to back
the top the forum index, at which point it is called
a “zombie thread”).
And though AFAIK it’s not officially frowned on, such
posts are usually followed by a lot of complaining, so
I thank you for your consideration, Coffee…
Ouch. And, agreed.
from a math PhD family member, with the caveat “anyone who’d understand this joke has already heard it.”
Q : What’s purple and commutes?
A: An abelian grape.
More group theory:
There was a young man named Galois
Who committed a giant faux pas.
By inventing Group Theory
he made our lives more dreary.
When they shot him, we shouted hurrah.
From my point of view, it seems more responsible to bring up and old topic then create a new one - to prevent the same things from being said. We all get a case of deja vu.
True, but the problem with zombie threads is that
people tend to click on them and spend a long time
reading what they think are new comments, often
from people they haven’t seen in awhile, only to
realize too late that they have been reading a 2-
year-old thread. Often it can get REALLY confusing
if the thread was about a current event for obvious
reasons. Of course, there are exceptions, like the
massive YouTube and CutiePie threads, so I guess
it’s often up to the poster around here.
If you’re going to tell math jokes, you have to be prepared for fastidious math geeks to correct you: “or equal to” should be removed, because zero squared is equal to zero, and zero is a real, non-imaginary number. 
Otherwise, I love the joke.
My old favorite math jokes are probably all on the ancient threads and don’t bear repeating. (And yes, I had heard the Abelian grape one before. ) I do remember that I won a whistle from Avanutria years ago by submitting this joke:
Q: Why do geeks confuse Halloween with Christmas?
A: Because OCT31 = DEC25
Caroluna, I can’t believe you would slander Galois and his theory like that! 
I don’t remember much of it anymore, but I rather LIKED Galois theory at the time I studied it. Didn’t the Zassenhaus Butterfly Lemma figure in it somewhere? You just gotta love anything with a name like that. (And no, I can’t remember what it says!)
I just read a wonderful math-history book that I highly recommend: The Poincare Conjecture, by Donal O’Shea. You DON’T need a graduate degree in math to understand most of it.
Long ago in China there were two clans.
One clan lived on the North side of the Tu river.
They called their clan “Thag”, which meant “The
People” in their language. The Thags were very
proud of the square in their village. It was a small,
but well-swept, and in the middle of the square was
a huge pot hanging from a very high frame. During
the summer months, the Thags would gather in the
square and make stone soup in their pot. It was
quite the feast, to be sure.
The other clan lived on the opposite side of the
Tu river. Their clan was called “Pai”, which meant “The
People” in their language. The Pai were often jealous
of their neighbors. Their village square was larger
than the Thag’s, but not so well-swept. The Pai had
also hung a cauldron on a frame in their square, but
had hung it too low, and so their pot could not hold
up to the heat of the cookfire. A hole eventually
formed in the bottom, and the pot was rendered
useless. That was the end of any stone soup making
in the Pai village square!
The people of the Pai village became increasingly
jealous of their neighbors’ stone soup gatherings, while
the Thags were envious of the other village square’s
size. Soon insults were hurled from either side of the
Tu. The Thag, seeing that violence was immenant,
talked their neighbors into mediation. The clans agreed
to engage a man that was respected by both clans: a
hermit known only as “the Korean”. Both clans agreed
to abide by the Korean’s decision. He toured both
villages, and finally came to a decision.
The elders of each clan came together to hear what
the Korean had to say. The hermit declared that every
summer, the two clans should share the use of the
cauldron in the Thag’s square to make stone soup,
and then gather both clans together in the larger
square in the Pai village to share in the bounty.
The Korean took a deep, contemplative breath and
declaimed: “Let it be remembered always that the
square with the high pot in use is equal, in summer,
to the square on the other Tu side.”
Even to this day, this saying has been known as the
Pai/Thag Korean Theorum.
Ouch…
That was really bad..
funny, very funny ..
Baaaad.
Slan,
D. 
How do different types of people prove that all positive integers are prime?
The mathematician: One is prime, two is prime, three is prime, therefore by induction, all positive integers are prime.
Statistician: One is prime, two is prime, three is prime, four is a statistical fluctuation, five is prime. Therefore, with a high degree of confidence, all positive integers are prime.
Biologist: One is prime, two is prime, three is prime, four is prime, five is prime, six is prime. . . .
Or how about a poem:
The chairman of the board has to choose a new CEO for the company so asks heads of various departments to com efor an interview.
He asks them all the same question; “What’s one plus one?”
The head of the accounting department says “two”.
The head of the mathematics department thinks for a long time and says “Approximately two.”
The head of the legal department gets up from his chair, closes all the blinds, comes back to the desk and says. “How much would you like it to be?”
Doc
Haha, I remember that contest. I think you won because it was both amusing and taught me something. Though whatever it was didn’t stick, apparently.
fearfaoin, I love that story. Through the whole thing I was trying to work out the math link but it still got me at the end.
It took many revisions to keep
from telegraphing the punchline.
There are 10 types of people in the world.
Those that understand binary
and those that don’t.
There are three kinds of people in the world: those who are good at maths and those who aren’t.
grimacing Ewwwwwww…bad one, Steve…
Not really a maths joke but it does have numbers in it.
Man goes to doctor. Doctor says to him, " I’m very sorry to have to tell you that you only have a very short time left to live."
“Oh no! Oh my God!! H…h…how long have I got, doc?”
“Ten.”
“Ten? TEN?? Oh God!! Ten what, doc? Months, weeks…days??”
“DON’T INTERRUPT ME! Nine, eight, seven…”
Excellent, Steve. Just what the doctor ordered! (At least for me.)
Sandy
This is a different spin on math
http://www.morenewmath.com/all/
And the Washington post also had a contest for similar equasions.
http://www.washingtonpost.com/wp-dyn/content/article/2009/04/03/AR2009040301196_2.html
How many mathematicians does it take to change a light bulb?
-e^(_i_π)