I don't have dyslexia but my keyboard doesn't contain either the 
UNION symbol or the INTERSECTION symbol (unless I want to go into an INSERT 
pull down menu every time I use those symbols). I don't need you to switch to 
English symbols, but I would like to see the English equivalents of the symbols 
you use (so that I can use them). I would also like a reference table defining 
each term in both your symbols and their English equivalents which I could look 
back to when I get confused. Please include examples. I tend to be somewhat 
careless when dealing with very fine distinctions and may type the wrong symbol 
while intending to type the correct one. Also, I must admit that the lessons 
are going too fast for me and are moving ahead before I've mastered the 
previous material. If I'm requesting too much simplification, please let me 
know because I'm quite well adjusted to my math disabilities and won't take 
offence at all. Thanks,      marty a.

  ----- Original Message ----- 
  From: Bruno Marchal 
  Sent: Thursday, June 04, 2009 12:04 PM
  Subject: Re: The seven step-Mathematical preliminaries 2

  Hi Marty,

  On 04 Jun 2009, at 01:11, m.a. wrote:

               I stopped half-way through because I'm not at all sure of my 
answers and would like to have them confirmed or corrected, if necessary, 
rather than go on giving wrong answers.   marty a.

  No problem.

      Exercise 1: Could you define in intension the following infinite set C = 
{101, 103, 105, ...}
      C = ?                          C={x such that x is odd & x <101}

  I guess you meant C = {x such that x is odd and x > 101}.  ">" means "bigger 
than", and "<" means little than. OK.

      Exercise 2: I will say that a natural number is a multiple of 4 if it can 
be written as 4*y, for some y. For example 0 is a multiple of 4, (0 = 4*0), but 
also 28, 400, 404, ...  Could you define in extension the following set D = {x 
⎮ x < 10  &  x is a multiple of 4}.    D=4*x  where x = 0 (but also 1,2,3...10)

  You cannot write D = 4*x ..., given that D is a set, and 4*x is a (unknown) 
number (a multiple of four when x is a natural number).

  Read carefully the problem. I gave the set in intension, and the exercise 
consisted in writing the set in extension. Let us translate in english the 
definition of the set D = {x ⎮ x < 10  &  x is a multiple of 4}: it means that 
D is the set of numbers, x, such that x is little than 10, and x is a multiple 
of four. So D = {0, 4, 8}.

  Indeed 0 is little than 10, and 0 is a multiple of four (because 0 = 4*0), and
  4  is little than 10, and 4 is a multiple of four (because 4 = 4*1)
  8 is little than 10, and 8 is a multiple of 4 (because 8 = 4*2)
  The next mutiple of 4 is 12. It cannot be in the set because 12 is bigger 
than 10.
  The numbers 1, 2, 3, 5, 7, 9 cannot be in D, because they are not multiple of 
4. You cannot write 1 = 4 * (some natural numbers), nor can you write 3 or 5, 
or 7 or 9 =  4 * x with x a natural number.

  Example: the set of multiple of 4 is {0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 
...}, all have the shape 4*x, with x = to 0, 1, 2, 3, ...
  The set of multiple of 5 is {0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 

      A ∩ B = {x ⎮ x ∈ A and x ∈ B}.

      Example {3, 4, 5, 6, 8} ∩ {5, 6, 7, 9} = {5, 6}

      Similarly, we can directly define the union of two sets A and B, written 
A ∪ B in the following way:

      A ∪ B = {x ⎮ x ∈ A or x ∈ B}.    Here we use the usual logical "or". p or 
q is suppose to be true if p is true or q is true (or both are true). It is not 
the exclusive "or".

      Example {3, 4, 5, 6, 8} ∪ {5, 6, 7, 9} = {3, 4, 5, 6, 7, 8}.   Question: 
In the example above, 5,6 were the intersection because they were the (only) 
two numbers BOTH groups had in common. But in this example, 7 is only in the 
second group yet it is included in the answer. Please explain.

  In the example "above" (that is {3, 4, 5, 6, 8} ∩ {5, 6, 7, 9} = {5, 6}) we 
were taking the INTERSECTION of the two sets.
  But after that, may be too quickly (and I should have made a title perhaps) I 
was introducing the UNION of the two sets.

  If you read carefully the definition in intension, you should see that the 
intersection of A and B is defined with an "and". The definition of union is 
defined with a "or". Do you see that? It is just above in the quote.

  I hope that your computer can distinguish A ∩ B  (A intersection B) and A ∪ B 
 (A union B).
  In the union of two sets, you put all the elements of the two sets together. 
In the intersection of two sets, you take only those elements which belongs to 
the two sets.

  It seems you have not seen the difference between "intersection" and "union". 
 I guess you try to go to much quickly, or that the font of your computer are 
too little, or that you have eyesight problems, or that you have some dyslexia.

      Exercice 3. 
      Let N = {0, 1, 2, 3, ...}
      Let A = {x ⎮ x < 10}
      Let B = {x ⎮ x is even}
      Describe in extension (that is: exhaustion or quasi-exhaustion) the 
following sets:

      N ∪ A = {0,1,2,3...} inter {x inter x<10}= {0,1,2,3...9}
      N ∪ B = {0,1,2,3....} inter {x inter x is even}= {0,2,4,6...}
      A ∪ B = {x inter x <10} inter {x inter x is even}= {0,2,4,6,8}
      B ∪ A = {x inter x is even} inter {x inter x < 10}= {0,2,4,6,8}

  All that would be correct if you were searching the intersection, but "∪" is 
the UNION symbol. (and "∩" is the INTERSECTION symbol).

  also you wrote the "⎮" as "inter", instead of "such that". 

      N ∩ A = {0,1,2,3...} inter {x inter x<10}= {0,1,2,3...9}
      B ∩ A =  {x inter x is even} inter {x inter x < 10}= {0,2,4,6,8}
      N ∩ B =  {0,1,2,3....} inter {x inter x is even}= {0,2,4,6...}
      A ∩ B =   {x inter x <10} inter {x inter x is even}= {0,2,4,6,8}

  All that is correct. Careful you were still using "inter" in place of "such 
that". Your last line should be

  A ∩ B =   {x such that x <10} inter {x such that x is even}= {0,2,4,6,8}

      Exercice 4

      Is it true that A ∩ B = B ∩ A, whatever A and B are?       yes
      Is it true that A ∪ B = B ∪ A, whatever A and B are?      yes

  Both are correct.

  Not bad Marty!  Just read carefully. I thing you have just dismiss the 
paragraph were I define "UNION". And then, you or your computer seems to have a 
trouble in distinguishing the symbols "∩" and "∪".  

  {1, 2, 3} ∩  {3, 4, 5} = {3}
  {1, 2, 3} ∪  {3, 4, 5} = {1, 2, 3, 4, 5}

  Tell me if it is OK, now.

  And then I let you think on the next exercises. Take the time to read slowly. 
Have you a problem of dyslexia? Do you see the difference between "<" and ">" ?
  If there is a problem with the symbols, I can switch on "english symbol".

  Have a nice week-end, don't hesitate to ask questions, clarification of 
points, or more examples.



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