Mathematicians are a bit strange ..

Or so they say.
I have known two mathematicians well, one was my teacher - who often said that infinity was not a number, it was a place - but who hit his wife over the head with a frying pan and killed her.
The other was a university friend who got annoyed one day when he couldn't open his door and punched it, breaking his hand in many places and taking the door, frame and all, down.
I digress. I write to ask, what is the ?Greek? letter which looks like a small p or Greek rho with the stem extended upwards? You see it on blackboards mostly.
Thanks
Peter Fairbrother