EXAMPLE. The set of points ðx; y; zÞ such that z ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 ðx2 þ y2Þ
p
comprises the surface of a hemisphere of radius
1 and center at ð0; 0; 0Þ.
For functions of more than two variables such geometric interpretation fails, although the terminology
is still employed. For example, ðx; y; z; wÞ is a point in four-dimensional space, and w ¼ f ðx; y; zÞ [or Fðx; y; z; wÞ ¼ 0] represents a hypersurface in four dimensions; thus x2 þ y2 þ z2 þ w2 ¼ a2 represents
a hypersphere in four dimensions with radius a > 0 and center at ð0; 0; 0; 0Þ. w ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
a2 ðx2 þ y2 þ z2Þ
p
,
x2 þ y2 þ z2 @ a2 describes a function generated from the hypersphere.