Question 176408
functions map inputs to unique outputs. A polynomial function of the first degree, such as y = 2x + 1, is called a linear function
All linear equations are functions with the exception of one type-- 
:
Use the vertical line rule to see if an equation is a function or not. If a vertical line intersects a plot at more than one point, the equation is NOT a function.
:
So, there is a linear equation that is not a function. Using the vertical line rule we conclude that a vertical itself is not a function. Maybe it would be easier to look at this from another definition of function. There is one and only one y value for each value of x. For x=a, there is infinite values of y. Thus a vertical line is not a function.

nonlinear function f(x) = x^2-1