Example of a function that has no inverse function?

Answer 1

y = sin x is such a function. It has an inverse, of course; but the inverse, sin-1, strictly speaking, is not a function.

Example: Given that x = pi/6, y must equal 0.5. However, given that y = 0.5, x can equal pi/6, 5 pi/6, 13 pi/6, 17 pi/6, or an infinity of values, both positive and negative.

For y to be a function of x, and x to be, also, a function of y, there must be exactly one value of y that answers to a given value of x, and vice-versa. Then, and only then, is each function the inverse of the other.