Maximize it!

Algebra Level 3

The maximum of

log ( ( x 5 10 x 3 y 2 + 5 x y 4 + 2 ) 2 + ( 5 x 4 y 10 x 2 y 3 + y 5 ) 2 ) \large \log \left(\sqrt{\left(x^5-10 x^3 y^2+5 x y^4+2\right)^2+\left(5 x^4 y-10 x^2 y^3+y^5\right)^2}\right)

in the closed unit disk can be written as log ( a ) \log (a) for some positive integer a a . Find a a .

Note that log ( ) \log(\cdot) above is the natural logarithm ( ln ( ) \ln(\cdot) ).


The answer is 3.

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1 solution

Hint : Writing z = x+i y, note that the function to maximize is log(Abs(2+z^5)).

As a bonus, note that this implies that the function to be maximized is harmonic (since 2+z^5 is nonzero and holomorphic in an open set containing the closed unit disk).

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