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## Dr. Bennette Harris – Problem Set 1

This Saturday’s (9/27) Math Circle lecture by Dr. Bennett Harris on “Computer Data Encryption – Decrypted” promises to be a great combination of number theory and applied math. To warm up the students, Dr. Harris forwarded some problems that I will post in two parts. Feel free to offer solutions in the comments or to just work them on your own. Full solutions will be made available at the next Math Circle.

### Problems

A solution for each of the following should either give the correct answer, or a technique for determining the answer in reasonable time with the assistance of a calculator.

2. What is the largest number you must test to demonstrate that 83 is prime?

3. What is the smallest 5-digit prime? How could you find this number?

4. Is 1234567890 prime? What about 123456789?

5. Assume an alphabet with 26 letters (plus a blank space). A substitution cipher is a one-to-one mapping of this alphabet onto itself. How many such substitution ciphers are there?

## Problem Solving Books

In addition to being the subject of books like Count Down, the Director of Metroplex Math Circle, Dr. Titu Andreescu is also the author of multiple books on problem solving. These books draw on his many years of experience as the director of AMC, coach of the US International Math Olympiad team and author of many contest problems.

To help the Metroplex Math Circle community we have created an Amazon List with some of Dr. Andreescu’s currently available books. In addition to Dr. Andreescu’s books for experienced problem solvers we have also included some books and resources on the list for students just starting into problem solving.

Not only does Metroplex Math Circle benefit from Dr. Andreescu himself, but many of his co-authors are also friends of MMC and frequent lecturers.

Following are the author descriptions from the book 104 Number Theory Problems: From the Training of the USA IMO Team: