Day 07 in Mat115

Today:

• Notice also that your project topics and partner names are due next time!

• Collect problems 2.1.

• Questions/Comments on old stuff?
  • Last time: we learned
    • What primes are,
    • How to find them using the Sieve of Eratosthenes,
    • How to decompose a number into its prime factors, and
    • That there are an infinite number of primes.

• Section 2.4: Crazy Clocks and Checking Out Bars:
  • Today is Wednesday, Sept. 14: on what day of the week will Christmas fall? (Sunday!)

  • If your birthday this year is on Monday, what day will your birthday fall on next year?

  • What is clock arithmetic?
    • What time will it be 27 hours from now?
    • What time will it be 27*24 hours from now?
    • What time will it be 25²⁴ hours from now?
    • What time will it be 30²⁴ hours from now?

  • What is 2³⁰⁰⁰ mod(7)? Why is this important? (Secret codes!)

  • What are UPCs? Universal Product Codes!
    • Break into groups to read samples.
    • Have groups provide the check digit for other groups' UPCs.

  • Why is correctly reading a UPC important? (computers read UPCs!)

  • What if a single digit is wrong in the UPC -- will the computer discover the error?

  • Problems from section 2.4: Crazy Clocks and Checking Out Bars:
    • #8, p. 90

• Lisp code to check UPCs.... Not for the faint of heart!;)