Graph of a function of two variables

Summary

• If $f:A→B$ is a function of two variables, then the set $(x,y,f(x,y)):(x,y)∈A$ is called the graph of $f$ (3-dimensional).
• If $f:A→B$ is a function of two variables, then the set ${(x,y):(x,y)∈A,f(x,y)=c}$ is called a level set or a level curve of $f$ or a graph of the equation $f(x,y)=c$ (2-dimensional).