# Nonlinear Optimization problem

Function $f(x) \in \mathbb{R}^n$, $(n\geq 1)$, depend on one parameter $x \in \mathbb{R}$. Performing a nonlinear transformation of $f(x)$, we obtain function $g(y) \in \mathbb{R}^n$. This transformation cannot be represented by analytical formulation. My optimization problem is to find $x$ in order $g(x) = C$, where C is explicit known.

What are possibilities (and best) strategy for solving this kind of optimization problem?

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I don't understand why you write $g(y) = C$. Is $g$ not a function of $x$? –  Christopher A. Wong Jul 3 '14 at 19:40
$g$ is not a function of $x$. But the transformation: $f -> g$ is unique. I mean find $x$ to $argmin||g(y)-C||$ –  user2863620 Jul 3 '14 at 19:52
You have not made it clear yet. If $g$ is not a function of $x$, then $g(y)$ has no dependence on $x$. –  Christopher A. Wong Jul 4 '14 at 0:21
I'm sorry. I mean the transformation is not explicit known by analytical formulation. I modified. –  user2863620 Jul 4 '14 at 7:19
Is $g$ continuous or piecewise continuous? Is it bounded above or below? If not, there isn't really any algorithm that's going to be guaranteed to work. –  Christopher A. Wong Jul 5 '14 at 6:00

What you have is an unconstrained, (presumably) very nonlinear, and possibly non-smooth optimization problem. By and large, the best you can do is to do an approximate Newton method where you numerically calculate the approximate slope, and then proceed to either the left or right of your current location depending on the sign. You can adjust how far you move (typically by powers of $2$) until you've successfully decreased your current value of $\|g(x) - C\|$ - this is called a damped Newton method.