CSC 201 - Discrete Math for
Computer Scientists
Professor Shai Simonson
Assignment 1
Due: By class time, Thursday, September 28.
Book Problems:
Section 4.2: Problems: 2a-c,
4a-d, 26, 46. Section 4.3: Problems 33c-d.
Section 4.4: Problems 2, 4, 9, 12, 37. Section
4.6: 24.
Section 5.1: Problems 6, 7, 11, 16, 18, 32, 49, 51.
My Problems:
- Prove by induction that 2n > n2 for all integers n larger than 4.
- Take any integer n > 1 and partition it in two
parts each greater than zero. For example 16 might be
partitioned into 3 and 13. Calculate the product of the
two numbers, in this case 39. Repeat this process with
each of the two numbers, until you are down to 1, keeping track
of the sum of all the products. For example, if you start
with 7, then you could get 3 and 4. Three becomes 1 and
2. Four becomes 2 and 2. The two's all become 1 and
1. This gives: 12 + 2 + 4 + 1 + 1 + 1 = 21.
Make a conjecture about the relationship of this sum to n. Prove your
conjecture by induction.
- Prove that the square root of 3 is irrational using
Aristotle/Hipassus’ techniques. Make sure to prove the
appropriate lemma.
- Compute by hand, the smallest positive integers x, y,
u,
v such that ax-by
= bu-av = gcd(a,b)
for each pair a, b below. Use
the Euclidean algorithm and back tracking. (i) 99,
101 (ii). 36, 42
- Prime Numbers: How many distinct divisors are there for
pa, where p is a prime?
Prove your conjecture by induction on a. How
many distinct divisors are there for an arbitrary number m? Hint: Factor m into its prime factors so that m
= p1a1p2a2…pnan
, and prove your answer by induction on n,
the number of distinct prime factors. Use the first part of this
question as your base case.
- Guess the number of different ways for n people to
arrange themselves in a straight line, and prove your guess is
correct by induction.
- Euclid proved that there is an infinite number of primes by
showing that if n is the highest prime, there is a
number larger than n (one plus the product of all the
primes through n) that is either prime
itself or has a prime factor greater than n. Find the
smallest n for which Euclid’s proof does not
provide an actual prime number. (You can write a program to find
it if you like).
- Guess a formula (or just look around the internet) for the sum
below, and prove you are right by induction.
1(2) + 2(3) + 3(4) + … + n(n+1)
- In Stonehill-ball, you can score 11 points for a goal, and 7
for a near miss. Prove by induction that you can achieve any
score greater than or equal to 60. Hint:
Try to add one point to your score by adding/subtracting
multiples of a and b. We did this in class
with 2 cent and 3 cent coins. Extra
Credit: Prove that in general that if a and
b are relatively prime, you can get any score greater
than or equal to (a-1)(b-1) where a and b are the two point values
(in our example a = 7
and b =11).
- RSA encryption. Let p=13
and q=11. Calculate an appropriate public code,
and private code for doing RSA encrypting.
Assuming that each character in a message is represented by its
ASCII value (an assigned table of integers from 0 to 127, can be
found in many texts), encode the message “Too much work!” by
encoding each ASCII value one at a time.
- If I want to send you something securely so that you know it
came from me and I know that only you received it, I can
put it in a box and lock the box keeping the only key in my
pocket. When you receive the box, you add your own lock to
it and send it back to me. I then remove my lock and
return the box once again to you, at which point you open your
own lock to get at the contents. Explain how to get this
effect electronically using RSA. Recall that RSA can be used for
encryption or authentication. Also, note that locking a
box is something anyone can do (public), and unlocking is
something only someone with the key can do.
back
shai@stonehill.edu
http://www.stonehill.edu/compsci/shai.htm