For problems 11-20, give a relational algebra expression to find the requested information from the relations defined below.

. Author(aid: int, name: string, dob: date) Book(bid: int, title: string, aid: int, price: real) Foreign key: aid references Author Customer(cid: int, name: string) Purchase(cid: int, bid: int, when : date) Foreign keys: cid references Customer, bid references Book 19. l'he usule of ev Hermann Hesse ery customer who has bought all of the books written by 20. The name of every author who has never purchased one of their own books