Björn Ulvaeus was a part of a Swedish pop group, formed in Stockholm in 1972, as well as co-producer on the film "Mamma Mia!" with who?
ABBA: ABBA (] ) were a Swedish pop group, formed in Stockholm in 1972 by members Agnetha Fältskog, Björn Ulvaeus, Benny Andersson, and Anni-Frid Lyngstad. Björn Ulvaeus:  He co-produced the film "Mamma Mia!"
i don't know