# How can I make logarithmic function first grow faster and then slow down faster?

How can I manipulate a logarithmic function to both grow faster as well as slow down faster?

I need to map a set of values to a range of real numbers between $0$ and $1$ (representing percentages by which to manipulate something). I am using this formula for it ($max > 0$, otherwise the formula would be more complicated):

$$f(x) = \frac {\log (1+x)}{\log (1+max)}$$

lowering the base of the log-function in the dividend makes $f(x)$ grow faster, but then $f(max) = 1$ is not true anymore.

What I am trying to accomplish is that smaller values in my set get mapped to higher percentage values, while higher values get only a slightly higher percentage-increase

• Could you explain, please, why does the lowering of the base of the log-function change $f(max)$? As far as I can see, $\frac{\log_n {(1+max)}}{\log_n {(1+max)}} = 1$ for any $n \in (0; 1) \cup (1; + \infty)$. – Yanior Weg Jul 6 '18 at 13:23
• Use $\frac{1}{x}$ ? – percusse Jul 6 '18 at 13:23
• hyperbolic tangent – Cato Jul 6 '18 at 13:49
• When you describe a function only qualitatively, it's hard to find. A differential equation cuts down the options considerably. Given some value of the function then fixes it. – Allawonder Jul 6 '18 at 13:52
• You can construct such function in a following way: choose any unbounded function $g: [0, +\infty) \rightarrow [0, +\infty)$, such that $g(0) = 0$ and $g(\frac{max}{x} - 1)$ is integrable on $[0, max]$. Then you can define $f(x) = \frac{\int_{[0, x]} g(\frac{max}{t} - 1)d\mu}{\int_{[0, max]} g(\frac{max}{t} - 1)d\mu}$. This function will satisfy the condition "that smaller values in my set get mapped to higher percentage values, while higher values get only a slightly higher percentage-increase", but if you want something more specific, please, do not hesitate to inform us about it. – Yanior Weg Jul 6 '18 at 14:09

You could add increase the additive constant $1$ in both logarithms or, equivalently, multiply both $x$ and $\max$ by some factor.