Vertical, horizontal line test
If a simple vertical line, is drawn in any point of f(x) , and this line intersects f(x) ONLY ONE point. Then this is a function.
Only one to one, ordered pair functions, have inverse.
Horizonta l line test determines, if a function have more than one, value for y, repeated.
If a function ,intersect a horizontal line more than once, then this function is not increasing, or decreasing uniformly.