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Math Help - Sum of perfect cubes

  1. #1
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    Sum of perfect cubes

    Prove there are infinitely many positive integers which cannot be represented as a sum of three perfect cubes

    hmmm how do you go about this question using contradiction?
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  2. #2
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    Okay, I'm not sure if this is the right way to go about this question, could someone please check it's validity.

    I conjecture that all perfect cubes are congruent to either

    Let be an integer.

    Then in we have:







    Now let be any perfect cubes.

    Thus

    However the least absolute residue is unaccounted for. Since there are infinitely many numbers congruent to then there are there are infinitely many positive integers which cannot be represented as a sum of three perfect cubes.
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  3. #3
    Kep
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    Usagi Killer is correct. Neat method!
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