The Institute of Mathematics is close to many hotels. Here is a short list
but many other at different prices can be found. Alternately, one can use booking.com or other travel sites.
Transportation is easy in Bucharest, by buses and metro, so a hotel not that close but at an affordble price can be a good choice.
We recommend the participants to book their hotel as soon as possible since Bucharest becomes more and more a touristic town.