A surge in economic activity stemming from massive public spending on construction of sports venues, housing, transportation and other infrastructure in the run up to the Games.The Games will  provide a material boost to economic activity.

Some are of the opinion that  the investment is just too small to generate a significant economic dividend …given the sheer size of the economy

A good article with som relevant references to the topic: http://journalistsresource.org/studies/society/culture/economic-and-cultural-benefits-of-the-olympics-research-roundup