Greatest common divisor

  1. Are there any real life applications of the greatest common divisor of two or more integers?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,259
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    Yes. i.e. whenever you have something depending on a ratio ...

    It's not normally expressed in that way though.
    Mostly - the lesson is important for the practise it gives in a kind of problem solving.
     
    Last edited: Jun 29, 2013
  4. There are a whole bunch. I use the idea regularly so it is difficult to point to a specific thing...
     
  5. Simon Bridge

    Simon Bridge 15,259
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    Aww go on - show us one... what's the one you use regularly that you last used?
     
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  7. rcgldr

    rcgldr 7,534
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    I can't think of any real world applications. The closest I can think of is the Euclid algorithm for finding the GCD which can be extended and used to find the inverse of a number in finite field, but it's seldom used because there are other and better methods. For example, if the field isn't very large, a lookup table can be used. In the case of hardware implementations of inversion based on "binary" finite fields (which is part of AES encryption), there are complex methods (sub-field mapping) that involve fewer gates than a lookup table. Wiki article for extended Euclid algorithm:

    wiki_inverse_in_finite_field.htm
     
    Last edited: Jun 30, 2013
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  8. GCD is used any time you want to simplify integers, but have the same ratios as the others have said. Another way to say it is the numbers scale equally. An example is in solving empirical formulas, where you reduce all integers in a chemical formula. Such as hexane C6H8 -> C3H4 GCD(6,8) is 2 so divide each by 2 to get the answer. Though it isn't of much use, but it has a name.
     
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  9. The store sells 8-packs of hotdogs and 12-packs of buns. If you want the same (nonzero) number of each, what's the cheapest way to do it?
     
  10. Simon Bridge

    Simon Bridge 15,259
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    @economicsnerd: good example, well done!
    Most people wouldn't do that by listing the divisors, but I suppose there are examples less amenable to a bit of trial and error.
    Do HS math text books no longer have examples like that these days?
     
  11. rcgldr

    rcgldr 7,534
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    Wouldn't this be the "lowest common multiple" as opposed to the "greatest common divisor"?
     
  12. Directly, I guess it's not really either.
    Computing either the GCD or the LCM would get you close to knowing how many bags of each to buy. You either compute {h/GCD(h,b), b/GCD(h,b)} or {LCM(h,b)/h, LCM(h,b)/b}.
     
  13. Stephen Tashi

    Stephen Tashi 4,389
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    If you can think of a real life application of linear Diophantine equations, then the GCD and the Euclidean algorithm have applications to solving those.
     
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