# Gentler transformation from real to positive

I am solving an equilibrium of an economic model for prices p > 0. I need to do this robustly for “wacky” parameter values when I am fitting the model to the data in an outer loop.

I have coded up a market clearing residual function market_residuals(model, p).

I found that I can do this by, roughly,

NLsolve.nlsolve(x0; autodiff = :forward, iterations = 200,
method = :newton, ftol = residual_tol) do x
p = exp.(x)
market_residuals(model, p)
end


The problem is that the Newton solver can very easily lead to arguments which are invalid when transformed, as

julia> exp(750)
Inf

julia> exp(-750)
0.0


Scaling x does not help as the Newton method is scale invariant.

Is there a “gentler” transformation from \Bbb{R} to \Bbb{R}_+ that one could recommend?

Maybe I don’t understand something, but can’t your function operate on logarithm of the price, so then simply skip call to exp?

That just puts the numerical problem somewhere else.

Maybe a trick is to use the identity (plus some offset) on positive numbers, and on negative numbers continue with something that plateaus, in such a way that the “junction” is still differentiable. For example the following:

f(x) = \begin{cases} x+\frac{\pi}{2} &\text{ if } x >0\\ \mathrm{arctan}(x) + \frac{\pi}{2} &\text{ otherwise.} \end{cases}

It should still be continuously differentiable, because the derivative of \mathrm{arctan} in 0 is 1.

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p²?

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I should have specified, but I need a bijection (continuous, monotone; does not need to be continuously differentiable but that would be nice, too).

Sigmoid takes you from R to in the interval between 0 and 1. Perhaps useful?

How to calculate sigmoid without exp(x)?

You can calculate it using either exp(x) or exp(-x) so you can choose one of them that works. But if one is too big I guess the other one would be too small…

f(x, slope, abcissa) = slope/2 * x + sqrt(slope^2/4 * x^2 + abcissa^2)


increasing abcissa should move the x where f(x)==0 to smaller values.

Why? Finding a root of f(p²) means that every root is doubled, but if Newton converges to a negative root you can just flip the sign.

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What about using log(1+exp(x)). Is monotonic, smooth, and never blows up

It still overflows because of the intermediate exp(x) calculation: log(1+exp(710)) === Inf. It’s also subject to underflow: log(1+exp(-37)) == 0.0. It would be better to use log1p(exp(x)), but even that eventually underflows: log1p(exp(-746)) == 0.0. You could define your own special-function implementation for f(x) = log1p(exp(x)), rewritten to avoid spurious overflow — for example, f(x) = x > 0 ? x + log1p(exp(-x)) : log1p(exp(x)) suffices and is equivalent to log(1+exp(x)) in exact arithmetic — but the underflow is unavoidable since f(-746) ≈ 1.03e-324 is not representable as a Float64.

I still don’t understand why you need a bijection in a root-finding context, e.g. why x -> x^2 is not acceptable, since you can flip a negative solution to a positive one a posteriori as I commented above.

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Translating by the minimum element of the set?

If the error is only happening inside the Newton solver (ie. it is not the actual market clearing price) could you do a small check beforehand to bound the admissible set of x and then use a bounded solver instead?

I usually try to do these domain transformations with bijections in Bayesian inference, but I realized that here there is no compelling reason for that, so I will experiment with your suggestion. Thanks for bringing it up again.

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