/* [wxMaxima batch file version 1] [ DO NOT EDIT BY HAND! ]*/
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/* [wxMaxima: comment start ]
Eric Doviak
26 April 2012
Math Methods 7025X
contemplate the slope on a saddle
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/* [wxMaxima: input start ] */
z(x,y) := x^2 - y^2;
plot3d(z,[x,-1,1],[y,-1,1])$
/* [wxMaxima: input end ] */
/* [wxMaxima: comment start ]
at the saddle point, the slope is zero along both the x and y axes (necessary condition)
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/* [wxMaxima: input start ] */
solx:subst(solve(diff(z(x,y),x)=0,x)[1],x)$
soly:subst(solve(diff(z(x,y),y)=0,y)[1],y)$
print("")$
print("the first order conditions")$
print("")$
print("d z(x,y)"/"d x"," = ",diff(z(x,y),x)," = 0")$
print("")$
print("d z(x,y)"/"d y"," = ",diff(z(x,y),y)," = 0")$
print("")$
print("imply that:")$
print("x = ",solx)$
print("y = ",soly)$
print("at the saddle point")$
/* [wxMaxima: input end ] */
/* [wxMaxima: comment start ]
at the saddle point, there is upward movement in both directions along one dimension
and there is downward movement in both directions along the other dimension
(sufficient condition)
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/* [wxMaxima: input start ] */
/* set up the Hessian matrix */
dxx:''(diff(diff(z(x,y),x),x))$
dyy:''(diff(diff(z(x,y),y),y))$
dxy:''(diff(diff(z(x,y),x),y))$
H:matrix([dxx,dxy],[dxy,dyy])$
print("")$
print("at a saddle point, one of the own-partials is positive and the other is negative:")$
print("")$
print("d^2 z(x,y)"/"(dx)^2"," = ",dxx)$
print("")$
print("d^2 z(x,y)"/"(dy)^2"," = ",dyy)$
print("")$
print("the cross-partial:")$
print("")$
print("d^2 z(x,y)"/"dx dy"," = ",dxy)$
print("")$
print("the Hessian matrix:")$
print("H = ",H)$
print("")$
print("determinant of Hessian is negative at a saddle point")$
print("det(H) = ",determinant(H))$
/* [wxMaxima: input end ] */
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