Accommodation
When trying to find a suitable accommodation You may begin with
Booking.com
See this map for the location of Mathematical Department.
Here is a list of some hotels in the neighbourhood of Stockholm University:

• Elite Hotel Arcadia
  
• Best Western
  
• Scandic Park Hotel
• Hotel Stallmastaregarden
• Elite Palace Hotel
  Few rooms are pre-booked at this hotel. The (fixed) price is 1890 SEK per night, which is much higher than you may get via Booking.com if You book in advance.
• Vanadis Hotel, (budget)
You may also look for some accommodation via Stockholm's official visitors guide

Visit Stockholm