Math Think

Hi there. I'm doing Stanford's Introduction to Mathematical Thinking. Following are my notes and assignment answers.

Resources

The course textbook is Introduction to Mathematical Thinking by Keith Devlin.

Lecture 1

Assignment 1

Question 1

Q: Find two unambiguous (but natural sounding) sentences equivalent to the sentence The man saw the woman with a telescope, the first where the man has the telescope, the second where the woman has the telescope.

A:

• The man with the telescope saw the woman
• The woman with the telescope was seen by the man

Question 2

Q: For each of the three ambiguous newspaper headlines I stated in the lecture, rewrite it in a way that avoids the amusing second meaning, while retaining the brevity of a typical headline:

1. Sisters reunited after ten years in checkout line at Safeway.
2. Large hole appears in High Street. City authorities are looking into it.
3. Mayor says bus passengers should be belted.

A:

1. After ten years apart, Sisters reunited in checkout line at Safeway.
2. Large hole appears in High Street. City authorities are investigating.
3. Mayor says bus passengers should wear seatbelts.

Question 3

Q: The following notice was posted on the wall of a hospital emergency room:

No head injury is too trivial to ignore.

Reformulate to avoid the unintended second reading. (The context for this sentence is so strong that many people have difficulty seeing there is an alternative meaning.)

A:

All head injuries are serious and require investigation.

Question 4

Q: You often see the following notice posted in elevators:

In case of fire, do not use elevator.

This one always amuses me. Comment on the two meanings and reformulate to avoid the unintended second reading. (Again, given the context for this notice, the ambiguity is not problematic.)

A: The notice is not suggesting that the elevator can be used to combat the fire.

If there is a fire do not travel using the elevator.

Question 5

Official documents often contain one or more pages that are empty apart from one sentence at the bottom:

This page intentionally left blank.

Does the sentence make a true statement? What is the purpose of making such a statement? What reformulation of the sentence would avoid any logical problems about truth? (Once again, the context means that in practice everyone understands the intended meaning and there is no problem. But the formulation of a similar sentence in mathematics at the start of the twentieth century destroyed one prominent mathematician’s seminal work and led to a major revolution in an entire branch of mathematics.)

Q: Does the sentence make a true statement?

A: No.

Q: What is the purpose of making such a statement?

A: That the otherwise blank page is there deliberately.

Q: What reformulation of the sentence would avoid any logical problems about truth?

A: This page intentionally left blank, sans this notice.

Question 6

Q: Find (and provide citations for) three examples of published sentences whose literal meaning is (clearly) not what the writer intended. (This is much easier than you might think. Ambiguity is very common.)

A: My answers taken from the book titles listed on the Wikipedia page of Professor Devlin:

1. Goodbye, Descartes: the End of Logic and the Search for a New Cosmology of the Mind
   • What has ended? Logic? Or Logic and the Search for a New Cosmology of the Mind?
2. Life by the Numbers
   • Life beside the numbers? Life per the numbers?
3. The Man of Numbers: Fibonacci's Arithmetic Revolution
   • Fibonacci rotated arithmetic 360 degrees? 6.2832 radians? What does that even mean?

Question 7

Q: Comment on the sentence "The temperature is hot today." You hear people say things like this all the time, and everyone understands what is meant. But using language in this sloppy way in mathematics would be disastrous.

A: It's the environment on the day which is hot; the temperature is a measure of the heat, not a substance which can be heated.

Question 8

Q: How would you show that not every number of the form $N=(p_{1}\cdot p_{2}\cdot p_{3}\cdot ...\cdot p_{n})+1$ is prime, where $p_{1},p_{2},p_{3},...,p_{n},...$ is the list of all prime numbers?

A: All that is required is one counter example. So I wrote a computer program to find one...

 function main() {

   let list = [];

   for ( let p of list_primes( 100 ) ) {

     console.log( p );

     list.push( p );

     var test = multiply( list ) + 1;

     if ( is_prime( test ) ) { continue; }

     return console.log( 'test ' + test + ' is not prime for p_n = ' + p );

   }
 }

 function multiply( list ) {

   var result = 1;

   for ( let n of list ) {

     result *= n;

   }

   return result;

 }

 function* list_primes( max ) {

   var n = 2;

   for ( ;; ) {

     if ( n >= max ) { break; }

     if ( is_prime( n ) ) { yield n; }

     n++;

   }
 }

 function is_prime( n ) {

   for ( var i = 2; i < n; i++ ) {

     if ( n % i === 0 ) { return false; }

   }

   return true;

 }

 main();

The output is:

2
3
5
7
11
13
test 30031 is not prime for p_n = 13