# Graph of a function of two variables

## Definition

Suppose  is a function of two variables , with domain a subset  of . The graph of  is a subset of three-dimensional Euclidean space  with coordinates , given by the equation:



Equivalently, it is the set of points:



Pictorially, this graph looks like a surface for a nice enough function .

Another way of defining the graph is that for every point , there is precisely one point of the graph on the line , namely the point with .

The - and -axes are the independent variable axes and the -axis, also called the -axis, is the dependent variable axis.

## Aspects

### Domain and range

Aspect of the function How it can be deduced from the graph
domain project the entire graph on the -plane.
range project the entire graph on the -axis.

### Vertical line test

The vertical line test for a function of one variable says that every vertical line intersects the graph in exactly one point if the -coordinate is in the domain and in no point if the -coordinate is not in the domain. There is an analogous test for a function of two variables. This says that any line parallel to the -axis (the function value axis) intersects the graph in exactly one point if the -pair for the line is in the domain, and intersects the graph in no point if the -pair for the line is not in the domain. In particular, any line parallel to the -axis must intersect the graph of a function in at most one point.

If a particular subset of  violates this condition, it cannot be realized as the graph of a function.

## Restriction to one variable

### Graph of the restriction

Suppose we fix  but allow  to vary. On the domain side, this is equivalent to considering the horizontal line  in the -plane. Suppose we are interested in studying the restriction of the function to this line (or more precisely, the intersection of this line with ). In other words, we are interested in studying the function:



This is a function of one variable, namely . Further, the graph of this function can be obtained by intersecting the graph of the original function with the plane  in . Note that this plane is parallel to the -plane. In this plane, we treat  as the independent variable and  as the dependent variable.

Analogously, suppose we fix  but allow  to vary. On the domain side, this is equivalent to considering the vertical line  in the -plane. Suppose we are interested in studying the restriction of the function to this line (or more precisely, the intersection of this line with ). In other words, we are interested in studying the function:



This is a function of one variable, namely . Further, the graph of this function can be obtained by intersecting the graph of the original function with the plane  in . Note that this plane is parallel to the -plane. In this plane, we treat  as the independent variable and  as the dependent variable.

### Continuity in each variable and separate continuity in graphical terms

We have the following:

Assertion about continuity How we can verify it from the graph
 is continuous in  at the point  Consider the graph restricted to the plane . This is continuous at .
 is continuous in  at the point  Consider the graph restricted to the plane . This is continuous at .
 is separately continuous continuous in both variables at the point . Both the above conditions.
 is continuous in  everywhere. The restrictions of the graph to all planes parallel to the -plane give graphs of continuous functions.
 is continuous in  everywhere. The restrictions of the graph to all planes parallel to the -plane give graphs of continuous functions.
 is separately continuous in both variables everywhere. Both the above conditions, i.e., the restrictions of the graph to all planes parallel to either the -plane or the -plane are graphs of continuous functions.

### Partial derivatives in graphical terms

For further information, refer: partial derivative

We have the following:

Partial derivative Graphical interpretation
The partial derivative  at a point  in the domain of the function The slope of the tangent line at  to the restriction of the graph of  to the plane .
The partial derivative  at a point  in the domain of the function The slope of the tangent line at  to the restriction of the graph of  to the plane .

### Directional derivatives in graphical terms

For further information, refer: directional derivative

The directional derivative  in the direction of a unit vector  at a point  can be determined as follows: first, intersect the graph of the function with the plane . This plane is perpendicular to the -plane and its intersection with the -plane is the line through  in the direction of the unit vector .

This intersection can be thought of as the graph of a function of one variable, where the point  is treated as the origin, the direction  is the independent variable axis, and the -axis direction is the dependent variable axis. Now, the directional derivative is the slope of this graph for dependent variable value of 0.

### Gradient vector in graphical terms

For further information, refer: gradient vector

We say that  is differentiable at a point  if the gradient vector exists at the point. This is equivalent to the graph of the function having a well defined tangent plane at . Further, the equation of this tangent plane is given by:



Another way of putting this is:



Note that it is possible that the partial derivatives both exist but the function is not differentiable. In this case, the surface does not have a well defined tangent plane at the point. Even though we can define a plane by the equation above, this is not the tangent plane, because the tangent plane does not exist.