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Question Number 2548 by Filup last updated on 22/Nov/15
For a function y=f(x),  inflection points/stationary points are  when  (df/dx)=0.    For a function z=f(x, y), can you find  these points through a similar method?    Is it something like (∂f/∂x)=0 and (∂f/∂y)=0?
Forafunctiony=f(x),inflectionpoints/stationarypointsarewhendfdx=0.Forafunctionz=f(x,y),canyoufindthesepointsthroughasimilarmethod?Isitsomethinglikefx=0andfy=0?
Answered by Yozzi last updated on 22/Nov/15
Let f be a function of two variables  whose first and second partial   derivatives are continous on some open  disc B. Suppose further that at the point  (a,b)  f_x =(∂f/∂x)=0   and   f_y =(∂f/∂y)=0.   Let p=(∂^2 f/∂x^2 )=f_(xx)  , q=(∂^2 f/∂y^2 )=f_(yy)  and r=(∂^2 f/(∂y∂x))=f_(xy)   (a,b) is a local minimum if pq−r^2 >0  and p>0 (or q>0),  (a,b) is a local maximum if pq−r^2 >0   and p<0  (or  q<0)  (a,b) is a saddle point if pq−r^2 <0  pq−r^2 =0 is inconclusive.
LetfbeafunctionoftwovariableswhosefirstandsecondpartialderivativesarecontinousonsomeopendiscB.Supposefurtherthatatthepoint(a,b)fx=fx=0andfy=fy=0.Letp=2fx2=fxx,q=2fy2=fyyandr=2fyx=fxy(a,b)isalocalminimumifpqr2>0andp>0(orq>0),(a,b)isalocalmaximumifpqr2>0andp<0(orq<0)(a,b)isasaddlepointifpqr2<0pqr2=0isinconclusive.
Commented by Filup last updated on 22/Nov/15
My question is  what is an open disc and what is f_(xy) ?  is  (∂^2 f/(∂x∂y))  differentiation with two variables?
Myquestioniswhatisanopendiscandwhatisfxy?is2fxydifferentiationwithtwovariables?
Commented by Yozzi last updated on 22/Nov/15
f_x  is partial differentiation of f w.r.t  x  f_(xy) =(∂/∂y)(f_x )=(∂/∂y)((∂f/∂x))=(∂^2 f/(∂y∂x)) (mixed 2nd partial derivative)  An n−dimensional open disc of  radius r is the collection of points less  than a distance of r away from a fixed  point in Euclidean n− space. (Wolfram Alpha)  If n=1 for example, this is an open   interval (s,t). So, for n=2 this   defines a collection of points in a circle containing  the point (a,b) where both f_x  and f_(y )   are zero. In this case, once the 1st  and 2nd partial derivatives of f exist  and the both f_(xy)  and f_(yx  )  are  continuous for the set of points (x,y)  in the disc, these two mixed partial  2nd derivatives are equal, i.e f_(xy) =f_(yx) .
fxispartialdifferentiationoffw.r.txfxy=y(fx)=y(fx)=2fyx(mixed2ndpartialderivative)AnndimensionalopendiscofradiusristhecollectionofpointslessthanadistanceofrawayfromafixedpointinEuclideannspace.(WolframAlpha)Ifn=1forexample,thisisanopeninterval(s,t).So,forn=2thisdefinesacollectionofpointsinacirclecontainingthepoint(a,b)wherebothfxandfyarezero.Inthiscase,oncethe1stand2ndpartialderivativesoffexistandthebothfxyandfyxarecontinuousforthesetofpoints(x,y)inthedisc,thesetwomixedpartial2ndderivativesareequal,i.efxy=fyx.
Commented by Filup last updated on 22/Nov/15
I see. Very interesting. I must go learn  multi−variable calculus!
Isee.Veryinteresting.Imustgolearnmultivariablecalculus!

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