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Math Help - Question about logic

  1. #1
    Newbie Grich's Avatar
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    Question about logic

    I has wondering what these mean in Logic:
    <br />
p \gets q~<br />
p \to q~<br />
p \leftrightarrow q<br />
    I've got an article on it but the explanation is too abstract.
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  2. #2
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    Hello, Grich!


    . . \begin{array}{ccc}p \gets q & \quad & \text{If }q\text{, then }p \\ \\[-3mm]<br />
p \to q & \quad & \text{If }p\text{, then }q \\ \\[-3mm]<br />
p \leftrightarrow q & \quad & p\text{ if and only if }q \end{array}


    . . . I hope that's clear enough.

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  3. #3
    Newbie Grich's Avatar
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    So,
    P \gets Q
    If Q is true then P is true, if Q is false then P is false
    P \to Q
    If P is true then Q is true, if P is false then Q is false
    P\leftrightarrow Q
    P is true when Q is true, P is false when Q is false

    Is this correct?
    Last edited by Grich; September 2nd 2008 at 09:16 PM. Reason: Formatting
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  4. #4
    Moo
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    Hello,
    Quote Originally Posted by Grich View Post
    So,
    P \gets Q
    If Q is true then P is true, if Q is false then P is false
    P \to Q
    If P is true then Q is true, if P is false then Q is false
    P\leftrightarrow Q
    P is true when Q is true, P is false when Q is false

    Is this correct?
    Actually, the 1st and 2nd are the same relation.
    P \gets Q is like Q \to P

    Then, there's something you should learn : if P \to Q and if P is false, then Q is true. This is weird but it's logic

    Also, see the truth tables : Truth table - Wikipedia, the free encyclopedia
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  5. #5
    Newbie Grich's Avatar
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    Okay, second attempt. So:
    <br />
P \gets Q<br />
    so if the value that the Implication arrow is pointing to is False and the value that the arrow is pointing from is true, then the answer would be false. Else it would be true.
    <br />
P \leftrightarrow Q<br />
    And if the values in P and Q are equal then it's true. Else it is false.
    COOL
    Did I get it right?
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  6. #6
    Moo
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    Quote Originally Posted by Grich View Post
    Okay, second attempt. So:
    <br />
P \gets Q<br />
    so if the value that the Implication arrow is pointing to is False and the value that the arrow is pointing from is true, then the answer would be false. Else it would be true.
    <br />
P \leftrightarrow Q<br />
    And if the values in P and Q are equal then it's true. Else it is false.
    COOL
    Did I get it right?
    Basically, yes, it's perfect

    But your wording is really weird lol
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  7. #7
    Newbie Grich's Avatar
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    Quote Originally Posted by Moo View Post
    Basically, yes, it's perfect

    But your wording is really weird lol
    Ha Ha Your not the first one to say that.
    I think it's worded unusually because I wrote it like I was programming in a computer language. Thanks Moo.
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  8. #8
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    The definition of the conditional ( p--->q) from everyday life is the following:

    A true event cannot imply a false event

    whilst a false event can imply everything i.e. true or false

    The following example it may help a Little:

    we say:

    if i go to London i will buy a car.

    Let , p= i go to London and q= i buy a car,then we have:

    case 1: i went to London and i bought a car.
    in this case p is true and q is true and i will be called a true man

    But what is..............(p---->q).......in this case true or false ???
    I say true, what do you say???

    case 2: i went to London and did not buy a car .
    in this case p is true and q is false and i Will be called a lier.

    Again what is .................(p----->q)..........in this case???
    I say false, what do you say???


    case 3: i did not go to London and did not buy a car.
    in this case p is false and q is false also. Will i be called a lier ??

    what is ........................(p--->q)....................in this case???

    case 4: i did not go to London but i bought a car.
    in this case p is false and q is true.Will i be called a lier???

    ...................................NO................................................
    BECAUSE i could have gone to a local agency and buy a car although
    i did not go to London.

    and in this case .................(p--->q) ....................is true

    So we see that the conditional : if i go to London i will buy a car can be broken down to 4 cases ( TT,TF,FT,FF) AND the out come can be only T or F.

    THAT loosely explains the definition of the conditional in symbolic logic

    Now the expression : if......then..... can be used in arguments as well,but
    there the implication becomes a logical implication

    ......................look for relevant threads........................................... .

    Finaly,what is : if 2+3=5 then 4+4=8??? true or false?????
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