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Section 2.1 Graphs

Given a function from a set DR2 with values in R we can plot its value on the x3-axis as shown in Figure 2.1. The resulting object is a surface, called the graph of f. Formally, it is the set
graph(f):={(x1,x2,f(x1,x2))(x1,x2)D}
in R3.
Figure 2.1. To obtain the graph of f plot f(x) above xD.
As an example consider the graph of f(x,y):=5y(1y2)(1/2x2)x in Figure 2.2.
Figure 2.2. Graph of f(x,y):=5y(1y2)(1/2x2)x.
If DRN we still call the set
graph(f):={(x,f(x))xD}RN+1
the graph of f over D, even if we cannot visualise it!