Thursday, 1 March 2018

linear algebra - Linearly Independent Real Numbers over mathbbQ



Let n be an integer greater than 1 and let n2R be the unique positive n-th root of 2. Show that the real numbers 1,(n2)2,,(n2)n1 are linearly independent over Q.



Using the definition of linear independence, the problem is equivalent to showing that
c1+c2(n2)2++cn1(n2)n1=0

has only the trivial solution c1=c2==cn1=0



I believe there is something special about each n2 being a unique positive n-th root of 2 that allows us to state linear independence. In particular, this implies that each (n2)i is linearly independent where i{0,2,3,,n1}.



I don't know if my intuition is correct and how to prove this linear independence rigorously.


Answer



Hint: if there were a linear dependence relation between these elements, then n2 would satisfy a (monic) polynomial of degree (after dividing by the highest nonzero coefficient). On the other hand, \sqrt[n]{2} is a root of X^{n} - 2, which is irreducible over \mathbb{Q} by Eisenstein at 2. Why is this first bit in contradiction with the second fact?


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