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The definition of the elasticity (used in economics a lot) of a function f(x) is $\frac{\mathrm{d} \log(f(x))}{\mathrm{d} \log(x)}$. Using the chain rule, it is easy to show that:

$$ \frac{\mathrm{d} \log(f(x))}{\mathrm{d} \log(x)} =\frac{\mathrm{d}\log(f(x))}{\mathrm{d}f(x)}\frac{\mathrm{d}f(x)}{\mathrm{d}x}\frac{\mathrm{d}x}{\mathrm{d}\log(x))} =\frac{x}{f(x)}\frac{\mathrm{d}f(x)}{\mathrm{d}x} $$

I couldn't find a way to input the derivative directly into Mathematica though. Is there a way?

Dt[ Log[ f[x] ], Log[x] ]

almost works, but not quite.

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  • $\begingroup$ Possible duplicate: mathematica.stackexchange.com/a/27393/4999 $\endgroup$
    – Michael E2
    Oct 5, 2020 at 18:28
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    $\begingroup$ A somewhat more direct coding of the formula works: Dt[Log[f[x]]]/Dt[Log[x]]. $\endgroup$
    – Michael E2
    Oct 5, 2020 at 18:30
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    $\begingroup$ That is an excellent tip Michael E2. Thank you very much. $\endgroup$ Oct 5, 2020 at 19:40
  • $\begingroup$ In[1804]:= D[Log[f[Exp[y]]], y] /. y -> Log[x] Out[1804]= (x Derivative[1][f][x])/f[x] also handles this. $\endgroup$ Oct 6, 2020 at 14:05
  • $\begingroup$ This also works: Dt[Log[f[x]], Log[x]] /. x -> E^Hold[Log[x]] // ReleaseHold . $\endgroup$
    – Akku14
    Oct 9, 2020 at 13:37

1 Answer 1

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You can use the ResourceFunction "ChainD" for this purpose:

ResourceFunction["ChainD"][Log[f[x]],Log[x]]

(x f'[x])/f[x]

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  • $\begingroup$ Thanks. Had not come across that (experimental) function before. Looks like there are lots of interesting things to play around with using it. $\endgroup$ Oct 5, 2020 at 19:45

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