Wednesday, 5 August 2009

analytic number theory - If 2xand 3x are integers, must x be as well?

It sure feels like something that should be related to Schanuel's conjecture. Note that this is equivalent to finding integers m and q such that x=fraclnm+2piiqln3

is not an integer but 2x is. [3x simplifies to m by construction].



Continuing this, let us actually compute 2x. If we split it into real and imaginary parts, a large messy expression ensues. But it naturally splits into 2 reasonable cases, depending on whether mgt0 or mlt0. Let's deal with the positive case first. We get
2x=mlog32left(cos(2qlog3(2)pi)+isin(2qlog3(2)pi)right)


For that to be an integer, it has to at least be real, but unless 2log3(2)q is an integer, the sin term will not be 0. For q=0, this is mlog32. We can rewrite that as 2log3m. But we assumed that m was a power of 3, so log3m (and thus x) is an integer.



For the mlt0, we get the slightly more complicated
2x=(m)log32left(cos(2(2q+1)log3(2)pi)+isin(2(2q+1)log3(2)pi)right)


Since mlt0, the first term is real, so we need 2(2q+1)log3(2) to be an integer for the sin term to disappear, which cannot happen.



Can someone find a flaw in my reasoning? I somehow expect so, as this did not seem difficult, and I would expect it to be if it's an open problem!

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