Calculus of Variation: Extremum & Further Variances

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SUMMARY

The discussion focuses on the calculus of variations, specifically the conditions for determining extrema of functionals. It establishes that if the first variation, δI, equals zero, the functional may represent a maximum, minimum, or saddle point. Further variations, such as δ²I and δ³I, are crucial for distinguishing between these cases. The participants reference the book "Introduction to Calculus of Variations" by Fox for a comprehensive understanding of second variations and their implications.

PREREQUISITES
  • Understanding of functional analysis and variational principles.
  • Familiarity with the concepts of maxima, minima, and saddle points in calculus.
  • Knowledge of the notation and implications of variations (δI, δ²I, δ³I).
  • Access to "Introduction to Calculus of Variations" by Fox for deeper insights.
NEXT STEPS
  • Study the implications of the first variation in functional analysis.
  • Learn about the second variation and its role in determining extrema.
  • Explore the concept of saddle points in the context of calculus of variations.
  • Review the provided resource on sufficient and necessary conditions for minima.
USEFUL FOR

Mathematicians, physicists, and students studying calculus of variations, particularly those interested in optimization problems and functional analysis.

LagrangeEuler
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If for some functional ##I##, ##δI=0## where ##δ## is symbol for variation functional has extremum. For ##δ^2I>0## it is minimum, and for ##\delta^2I>0## it is maximum. What if
##δI=δ^2I=0##. Then I must go with finding further variations. And if ##δ^3I>0## is then that minimum? Or what?
 
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Finding further variations is useless from this point of view.
The stationarity of the functional, i.e. δI=0 , occurs for maxima, minima and saddles.
 
So then how I could know? Is it minimum or maximum?
 
Here you will find a better explanation than I could give on sufficient and necessary conditions for minima http://www.math.utah.edu/~cherk/teach/12calcvar/sec-var.pdf
If you have the book "introduction to Calculus of Variations" by Fox you will find there a thorough discussion of the second variation: yes further variations are to be computed.
I really do apologise for my previous reply which was wildly inaccurate due to a misunderstanding of mine.
 

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