- #1
Felafel
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Homework Statement
find the minima and maxima of the following function:
##f:\mathbb{R}^3 \to \mathbb{R} : f(x,y,z)=x(z^2+y^2)-yx##
The Attempt at a Solution
after computing the partials, i see ∇f=0 for every point in the x-axis: (a, 0, 0)
The Hessian is:
( 0 0 0 )
( 0 2a -1 )
( 0 -1 2a )
for every value of a, the determinant is 0.
the eigenvalues are: ##\lambda_1=0##
##\lambda_{2,3}= 2a±\sqrt{1}##=##2a\pm1 ##
##\lambda_{2,3}## are both positive if a>1/2 and both negative if a<-1/2.
Thus, for -1/2<a<1/2 i can say the points are inflection points, because the eigenvalues have oppisite sign.
But how about ##a \geq 1/2## and ## a \leq -1/2##? i get two positive/negative eigenvalues, but the first one is always zero, so i can't really say that they are maxima/minima, right? what's the method to determine if they are max/min? thank you very much
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