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, m string) Purchase(cid: int, bid in, when: date) - Foreign keys: cid references Customer, bid references Book 11. The name of every author over 50 12. The name of every author who is also a customer 13. The title of every book written by Margaret Atwood 14. The name of every customer who has bought a book for $20.00 or more 15. The title of every book that has never been purchased 16. The book title and date for every time a book was purchased on its author's 50th birthday 17. The name of every customer who has bought a book written by Hermann Hesse 18. The name of every customer who has bought at least two different books written by Hermann Hesse 19. The name of every customer who has bought all of the books written by Hermann Hesse 20. The name of every author who has never purchased one of their own books