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[Algebra] Proving equations involving modulo functions.

  1. Mar 12, 2015 #1
    I would like to know some general properties of the modulo (remainder) function that I can use to rewrite expressions. For example, say we wanted to prove the following by rewriting the right-hand-side:

    $$ \Big{\lfloor} \frac{n}{d} \Big{\rfloor} = \frac{n - n \pmod d}{d} $$

    I have no idea how to prove this statement formally. Intuitively, ##n \pmod d## is the remainder of whatever ##\big{\lfloor} \frac{n}{d} \big{\rfloor}## is, and therefore

    $$ n \pmod d = n - d \Big{\lfloor} \frac{n}{d} \Big{\rfloor}$$

    Which, when we insert it into the right-hand side of what I'm trying to prove, results in:

    $$ \frac{n - (n - d \big{\lfloor} \frac{n}{d} \big{\rfloor})}{d} $$

    Which, thankfully, boils down to:

    $$ \Big{\lfloor} \frac{n}{d} \Big{\rfloor} $$

    What I want to know is: How do I formalize the beginning of this proof? Is there some general property or definition of the modulo function which I can invoke to justify that step?
  2. jcsd
  3. Mar 12, 2015 #2


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    Homework Helper

    a = n mod d is defined as a number between 0 and d such that there exists an integer c such that n = cd+a.
    When you take d*floor(n/d), you are essentially saying that floor(n/d) = c. That is, there is no integer larger than c such that cd is less than or equal to n.
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