björn kristian ulvaeus (credited as björn ulvæus ; born 25 april 1945) is a swedish songwriter, producer, a former member of the swedish musical group abba (1972–1982), and co-composer of the musicals "chess", "kristina från duvemåla", and "mamma mia!". he co-produced the movie "mamma mia!" with fellow abba member and close friend benny andersson.