What's the context? You could try a \mathcal{H}, but that looks like this:
Is it in a math environment? Could you maybe post a scan of the page in which you find it?
Yes, it's in math context. It appears in Problem 7 on this page: http://www.math.upenn.edu/~cle/amcs609/ps3.pdf
Well, the Ralph Smith font is close. That's as close as I can get at the moment, without a lot more searching.