Inverting the sum of two logarithms?

43 Views Asked by At

It is well known how to invert the following relation to get $y$ in terms of $x$

$$x=a\log(y-b),\quad\to\quad y=b+\exp(x/a)$$

Is there anything that can be said about inverting the relation

$$x=a\log(y-b)+c\log(y-d)$$

For general values of $a$, $b$, $c$, and $d$?

When $a=-c$, this is straightforward to invert

$$y=\frac{b+d\exp(x/a)}{\exp(x/a)-1}.$$

Also when $a=c$, we find

$$y=\frac{b+d\pm\sqrt{(b-d)^2+4\exp(x/a)}}{2}.$$

I'd like to understand how to invert the double logarithm relation in general. Perhaps there is a well known special function which allows one to write the general inversion formula?

Update

Taking the advice of Dasherman, we find

$$\exp(x)=(y-b)^a(y-d)^c.$$

This looks difficult in general to invert. The other special case I neglected is when $b=d$,

$$y=b+\exp(x/(a+c))$$