Discovering an error after publishing? My paper was published 6 months ago. A few days ago, while reading it I discovered that one of the equations had the wrong power: instead of x it should have x². After checking my old notes it turns out that they had x², so its just a typing error. I should point out that this doesn't change the end result and the equation before and after it are both correct. 

What should I do? Should I tell the journal about this mistake?
This is the sort of thing that corrections and errata are for. The journal should have a procedure for informing the editorial staff and correcting the record.  Follow it, and you should be able to fairly quickly and easily correct the record.