genius-common 1.0.23-3 / usr / share / genius / examples / peano-restrictions.gel

# Category: Calculus
# Name: Peano surface (restrictions to lines)
#
# The Peano surface is the graph of the function.
#
# z = (y-x^2)(2x^2-y)
#
# The function giving the surface has a strict maximum on every line through
# the origin, but the origin is not an extremum of the function.
#
# In this example we draw the graphs of the restrictions to lines through
# the origin so that the maxima can be more clearly seen
#
# Note that the second derivative test fails on the line y=0, even though
# along that line the function still has a maximum.
#
# The properties of the function can be easily seen from the factorization
# and seeing the regions where each factor is positive or negative.
#
# If you play around with the vertical axis limits below you can see the
# maxima better

function f(x,y) = (y-x^2)*(2*x^2-y);

LinePlotWindow = [-1,1,-0.1,0.1];
#LinePlotWindow = [-1,1,-0.01,0.01];
#LinePlotWindow = [-1,1,-0.001,0.001];
#LinePlotWindow = [-1,1,-0.0001,0.0001];


LinePlotDrawLegends = false;

for theta=0.0 to 20*pi by 0.01 do (
	LinePlot(`(t)=f(cos(theta)*t,sin(theta)*t))
);