1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Discrete math - simple formalism question

  1. Aug 23, 2012 #1
    I never used descrete math terms in english before, so I hope it sounds clear enough:

    Formalize the following:
    1) Between every two different real numbers there is a rational number
    2) There exist real numbers x and y, such that x is smaller than y, yet x^2 is bigger than y^2

    Now the solution I wrote for 1 is:

    [itex]\forall x,y \in R.x\neq y \hspace{5 mm} \exists z\in Q.\left\langle[(x<z)\wedge (y>z)]\vee [(y<z)\wedge (x>z)]\right\rangle[/itex]

    Only then I checked the solution given by my teacher said:

    [itex]\forall x,y \in R.x\neq y \Rightarrow \exists z\in Q.\left\langle[(x<z)\wedge (y>z)]\vee [(y<z)\wedge (x>z)]\right\rangle[/itex]

    Now, where did this implication arrow come from, and is it necessary? If so, what's wrong with my solution then?

    For the second sentence we didn't get any solution, so I just want to confirm the following is correct:

    [itex]\exists x,y\in R.[(x<y)\wedge (x^2>y^2)][/itex]

    Thanks in advance for your time :)
  2. jcsd
  3. Aug 23, 2012 #2
    Re: Descrete math - simple formalism question

    The 'implies' is there to emphasize the logical connection there. If you have two reals like that, then you can find such a z. Moreover, if you can't find such a z, then x=y (or one of x,y is not a real number, which seems less likely).
    I probably would have left out the arrow as well, since the question is phrased as a statement, and not an implication. In any case, the sentence you wrote would usually be acceptable, but since your prof. wants the arrow in, leave it.

    Your second answer looks fine to me, unless you want to put a little "s.t." in between there.
  4. Aug 27, 2012 #3
    Hi gustav
    Thanks for your reply
    Could you just explain what "s.t." means? I'm not very familiar with the english terms.
    Thanks a bunch
  5. Aug 27, 2012 #4


    User Avatar
    Science Advisor

    s.t. = 'such that'. "There exist x and y such that..." means there exist x and y having the following property.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook