There are lots of websites, such as StubHub, that let people resell tickets to sports games and other events. But it can be nerve-wracking and expensive to buy those tickets, since you don’t know what’s going to happen to ticket prices. Should you buy now before prices go up further? Or will prices plummet right before an undersold event? SeatGeek, which demonstrated its service at the TechCrunch50 conference in San Francisco, says it has built a complex mathematical model that predicts how ticket prices will fluctuate on secondary ticket markets. It’s still in testing, but by looking at factors like weather, ticket prices at similar events, and other data, SeatGeek says that when predicting whether prices will rise or fall, it is already 75 to 80 percent accurate. The site is opening to the public today. It takes a 7 to 10 percent cut of each ticket sale, and offers other services like email alerts when prices hit their lowest point.