The normal answer is "no, there is no inverse function in terms of the normal operators and trigonometric functions." That is to say, given a value of x, you cannot find the value of y of the input function reflected over the x-y axis using standard functions. "Standard functions" is what is getting me here. All functions we can run on a computer is eventually compiled down to complex loops of additions, multiplications, subtractions, divisions, bit shifts, reads, and writes. I wish there was a reflect( <expression> ) operator, that just returned the value you would get if you flipped <expression> over the xy axis first. More to the point, sin(x) + x does not even fail any horizontal line tests. How can you determine that there is no inverse?