# July 01, 2020

## Bounded Variable Sigmoid Mapping Function for Curving Random Numbers

So the other day I wrote about a curving function for random numbers that can weight them more toward high or low values. What if we want numbers more concentrated at a center? We can use a Box-Muller transform and get numbers with Gaussian distribution. The problem with that is the numbers are unbounded. That is, there is no limit to how much derivation from the center there can be. We want a fixed upper and lower limit. I've tried doing this in the past with a ranged Box-Muller transform but it isn't quite as configurable as I would like. Then I ran across this post on Stack Exchange. It is a variable Sigmoid function bound between 0 and 1. Here is an implementation:

This is a fantastic function. For coefficient values less than 1 and greater than zero, the function exhibits Gausian distribution. This produces numbers around a center point (0.5) that can deviate to some maximum on either side (0 or 1) with control over how likely deviation (something akin to standard deviation). However, it is more versatile than that. For coefficients greater than 1, the function produces reverse Gaussian distribution, pushing the results to the sides and making the middle less likely. There are applications for both. However, let's first look at the transform function. The Stack Exchange post gives it as:

But I like this form better as it is easier to read:

It is a bounded function but is actually undefined at the end-points. I could see why *x*=0 didn't work (divide-by-zero), but it wasn't initially obvious why *x*=1 didn't work. This identity makes it more obvious:

So if *x*=1, the value inside the parenthesis becomes 0, and you cannot take the natural log of 0 as it is undefined at 0.

Looking at the function it is easy to deduce that at the end point *y*=0 when *x*=0 and *y*=1 when *x*=1, but we can prove it mathematically:

So a complete definition would be:

Now there must be a relation between the coefficient in this equation and the standard deviation. Sadly, I have was unable to find it before publishing this article.