Thursday, 8 January 2015

proof verification - Is there a pair of complex numbers for which |xy|>|x||y|?



This is part of larger program involving functions that preserve order over the positive reals.



But I'm trying to show that




x,yC |xy||x||y|



My work:



Let x=ax+bxi, Let y=ay+byi, =Real Part,=Imaginary Part



|xy|=|eln(x)y|=|eln(ax+bxi)(ay+byi)|=e(ln(ax+bxi))ay(ln(ax+bxi))by



This needs to be compared to |x||y|=eln(a2x+b2x)a2y+b2y




But i'm at a loss for how to compare these two objects. I do have some intuition though, for one as by the first expression tends towards 0, while the bottom expression grows without bound.


Answer



ii=exp(ilni)



Take the principal branch of the logarithm, then lni=iπ/2, thus



ii=exp(π/2)



and the requested inequality is satisfied as |x|=|y|=1 and |rhs|>1.



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