User:Algebraic

You can call me Al.

Let ${\displaystyle n_{i}(t)}$ be the number of organisms of type ${\displaystyle i}$ at time ${\displaystyle t}$, and let ${\displaystyle R}$ be the per-capita reproductive rate per generation. If ${\displaystyle t}$ counts generations, then

${\displaystyle n_{i}(t+1)=n_{i}(t)R\!}$

and

${\displaystyle n_{i}(t)=n_{i}(0)R^{t}.\!}$

Now we wish to move to the case where ${\displaystyle t}$ is continuous and real-valued. As before,

${\displaystyle n_{i}(t+1)=n_{i}(t)R\!}$

but now

${\displaystyle n_{i}(t+\Delta t)\!}$	${\displaystyle =n_{i}(t)R^{\Delta t}\!}$
${\displaystyle n_{i}(t+\Delta t)-n_{i}(t)\!}$	${\displaystyle =n_{i}(t)R^{\Delta t}-n_{i}(t)\!}$
${\displaystyle {\frac {n_{i}(t+\Delta t)-n_{i}(t)}{\Delta t}}}$	${\displaystyle ={\frac {n_{i}(t)R^{\Delta t}-n_{i}(t)}{\Delta t}}}$
${\displaystyle {\frac {n_{i}(t+\Delta t)-n_{i}(t)}{\Delta t}}}$	${\displaystyle =n_{i}(t){\frac {R^{\Delta t}-1}{\Delta t}}}$
${\displaystyle \lim _{\Delta t\to 0}\left[{n_{i}(t+\Delta t)-n_{i}(t) \over \Delta t}\right]}$	${\displaystyle =\lim _{\Delta t\to 0}\left[n_{i}(t){\frac {R^{\Delta t}-1}{\Delta t}}\right]}$
${\displaystyle {\frac {dn_{i}(t)}{dt}}}$	${\displaystyle =n_{i}(t)\lim _{\Delta t\to 0}\left[{\frac {R^{\Delta t}-1}{\Delta t}}\right]}$
${\displaystyle {\frac {dn_{i}(t)}{dt}}}$	${\displaystyle =n_{i}(t)\ln R\!}$

where the last simplification follows from L'Hôpital's rule. Explicitly, let ${\displaystyle \epsilon =\Delta t}$. Then

${\displaystyle \lim _{\Delta t\to 0}\left[{R^{\Delta t}-1 \over \Delta t}\right]}$	${\displaystyle =\lim _{\epsilon \to 0}\left[{\frac {R^{\epsilon }-1}{\epsilon }}\right]}$
	${\displaystyle =\lim _{\epsilon \to 0}\left[{\frac {{\frac {d}{d\epsilon }}\left(R^{\epsilon }-1\right)}{{\frac {d}{d\epsilon }}\epsilon }}\right]}$
	${\displaystyle =\lim _{\epsilon \to 0}\left[{\frac {R^{\epsilon }\ln R}{1}}\right]}$
	${\displaystyle =\ln R\lim _{\epsilon \to 0}\left[R^{\epsilon }\right]}$
	${\displaystyle =\ln R\!}$

The solution to the equation

${\displaystyle {\frac {dn_{i}(t)}{dt}}=n_{i}(t)\ln R}$

is

${\displaystyle n_{i}(t)=n_{i}(0)e^{t\ln R}=n_{i}(0)R^{t}.\!}$

Note that the continuous case and the original discrete-generation case agree for all values of ${\displaystyle t}$. We can define the instantaneous rate of increase ${\displaystyle r=\ln R}$ for convenience.