what other bands/singers has he worked with?
Britney Spears'
Max Martin
He rose to prominence in the late 1990s after making a string of hit singles such as Britney Spears' "...Baby One More Time" (1998), the Backstreet Boys' "I Want It That Way" (1999), and NSYNC's "It's Gonna Be Me" (2000).

who wrote shake it off by taylor swift Taylor Swift, Max Martin and Shellback. who is this second person mentioned? Karl Martin Sandberg, known professionally as Max Martin, is a Swedish singer, songwriter, and record producer
wiki:13083667
Max Martin [SEP] Introduction
Martin is the songwriter with the third-most number-one singles on the chart, behind only Paul McCartney (32) and John Lennon (26). In addition, he is tied for the most Hot 100 number-one singles as a producer, 23, along with George Martin, who had achieved 23 by the time of his death. In early 2013, his single sales were tallied by "The Hollywood Reporter" to be at over 135 million. According to "Variety", his net worth was approximately $260 million in 2017. The previous year he achieved approximately an income of $54 million and a profit of $19 million for his services.