How do I solve the following, analytically?


It looks like it should be simple, but whether I take the $e$th root of each side or take the $\log$ of each side (ending up with a $\log\log$), I get stuck—and both approaches seem naïve, anyhow.

It appears to have two solutions:

enter image description here

In case it raises some eyebrows, I only stumbled across the formula while blindly tinkering with reward functions for a productivity gamification system. In short, I needed a function $g(x)$ that behaves very similarly to $f(x)=x$ below a certain constant, e.g. 27 (which I can scale arbitrarily), then tapers off and flatlines. $g(x)=\log^e{(x+1)}$ happened to work perfectly, but of course I wouldn't assert the formula to be meaningful in any natural sense.

  • $\begingroup$ The latter; the $e$th power. $\endgroup$ – Andrew Cheong Sep 29 '14 at 9:29
  • $\begingroup$ @NajibIdrissi From the tries he put, I am convinced this is the $e$-th power of log, not the log in base $e$ $\endgroup$ – Martigan Sep 29 '14 at 9:29

Try the Lambert function, this is for such relations: http://en.wikipedia.org/wiki/Lambert_W_function

  • $\begingroup$ Would you elaborate? I don't see a way to apply the $\mathrm{W}$ function here. $\endgroup$ – robjohn Sep 29 '14 at 12:43
  • $\begingroup$ And if you take the exponentials of the terms? $\endgroup$ – Karl Sep 29 '14 at 17:42
  • 1
    $\begingroup$ Try it; it doesn't work. Elaboration is more than asking another question. $\endgroup$ – robjohn Sep 29 '14 at 20:24

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