Thursday, 8 January 2015

proof verification - Is there a pair of complex numbers for which $|x^{y}| > |x|^{|y|}$?



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



But I'm trying to show that




$$ \forall x,y \in \mathbb{C} \ |x^{y}| \le |x|^{|y|} $$



My work:



Let $x = a_x + b_xi$, Let $y = a_y + b_yi$, $\Re = \text{Real Part}, \Im = \text{Imaginary Part}$



$$ |x^{y}| = |e^{\ln(x)y}| = |e^{\ln(a_x + b_xi)(a_y+b_yi)}|=e^{\Re(\ln(a_x + b_xi))a_y-\Im(\ln(a_x + b_xi))b_y }$$



This needs to be compared to $|x|^{|y|} = e^{\ln \left(\sqrt{a_x^2 + b_x^2} \right)\sqrt{a_y^2+b_y^2}}$




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


Answer



$i^{-i}=\exp(-i \ln i)$



Take the principal branch of the logarithm, then $\ln i=i\pi/2$, thus



$i^{-i}=\exp(\pi/2)$



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



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