[switters]

I'm not quite sure what you're asking - are you trying to figure the probability of *someone* having top pair when n players see the flop? The problem with this sort of question is that it is much more likely that someone has top pair on a board like A45 than on a board like 743... you have to assume *something* about your opponents' play, i.e. that there are many more hands containing an A that they would play than hands containing a 7 that they would play.

If you had infinite time and a personal computer to use during the hand, you would figure this by putting each opponent on a range of hands, each with some % likelihood, and then you would add up all the hands that make top pair with the board, and divide by the total... unfortunately, you won't have this available to you, so you need to "ballpark" it.

so, if you have a hard time ever throwing away QQ to a Kxx flop with some betting action, you will probably give away money to top pair. on the other hand, if you auto-muck A6 to an 863 flop, you will probably also be giving away money, since your opponents will have top pair less often here... the answer, as always, is "it depends" - on the flop, the texture of the game, and what you know about your opponents' play.

-switters