Not a function: “one to many“. In other words, let’s say you had one x-value that maps to many y-values. For example, — in coordinate notation — (2,1) and (2,10). If the first number (the x-value) repeats, then you do not have a function. To put that another way, ...
Volumes grow according to the square of the hypersine ##(\frac{e^x - e^{-x}}{2})^2## The hypersine has a nice symmetry which the ordinary exponential function ex does not have. If you flip it right to left, over the y-axis, and then flip it top to bottom over...