I was reading an interesting paper, and early in the introduction there was an equation with a typo in it. I am absolutely sure of this, and all it was missing was a factor of $n!$. Overall, it was an inconsequential part of the paper, and doesn't affect anything, as it was in the section which tells the reader about the background of the area. But still, the equation as written is not correct, and could be confusing to someone reading who is trying to learn about the area.

In fact, the line right before this particular equation said "it is easily observed that..." which is a little interesting.

My question is: Should I do anything when I see this? Should I just ignore it, or is it polite to email the author? Or is it impolite to do so? If you published a paper, with an inconsequential, but possibly confusing typo in the introduction, would you want someone to tell you about it or just leave it alone?

Thanks for the advice!

Also please note, I am not referring to a spelling typo, that is definitely not important! Its just possible that this typo could confuse someone reading the paper. (I was confused at first!)