Determining the type of critical point when the second derivative test fails

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SUMMARY

The discussion focuses on determining the type of critical point for the function f(x,y) = x²y + xy² at the critical point (0,0). The second derivative test fails because all second derivatives are zero at this point. Participants suggest alternative methods, including using Taylor expansion and analyzing the behavior of the function along specific lines (x=0, y=0, y=-x). A recommended approach is to create a table of values to compare the signs of the first derivatives in the vicinity of the critical point, similar to the single-variable case.

PREREQUISITES
  • Understanding of critical points in multivariable calculus
  • Familiarity with second derivatives and their significance
  • Knowledge of Taylor series expansion
  • Basic skills in analyzing functions of two variables
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  • Learn how to apply Taylor series for functions of multiple variables
  • Research techniques for analyzing function behavior along specific paths
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kevinlightman
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Homework Statement



For example with f(x,y) = x2y + xy2

Homework Equations





The Attempt at a Solution



Well I know there is a critical point at (0,0). So I calculated the second derivatives but they are all 0 here so that doesn't help.
I also tried using the Taylor expansion to show that f(x,y)>f(0,0) or not but that didn't get me anywhere.
Then I tried considering the type of critical point on x=0,y=0,y=-x etc. but again I didn't get anywhere.
Any ideas?
 
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Draw up a table of values to compare values, and most importantly the signs, of the derivatives in the neighborhood of the critical points. It is the same reasoning as is the single variable case eg When we conclude that a critical point was a local maximum if the derivative from the left is positive and the derivative from the right is negative.
 

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