MHB Partial fractions ( part of a logistic equation)

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The discussion focuses on finding the partial fraction decomposition of the expression 1/N(k-N). The user initially identifies A as 1/K but struggles to determine the value of B. Other participants clarify that both A and B equal 1/K, leading to the complete decomposition of 1/(K*N) + 1/(K*(K-N)). Additionally, it's noted that the logistic equation can be approached as a special case of the Bernoulli equation, providing an alternative solution method that may be beneficial for the user. Overall, the thread emphasizes the importance of understanding both partial fractions and the logistic equation's broader mathematical context.
adzix
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Hi everyone, I am stuck on a problem. I need to give a partial fraction of 1/N(k-N). I have tried every method so far ( plotting roots, systems of equations). I think I found A=1/k but I have no clue how to find B value. I would really appreciate any help as I am a desperate student trying to pass the class. (the initial problem is dN/dt=(r/K)N(K-N), N(0)=No)
 
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adzix said:
Hi everyone, I am stuck on a problem. I need to give a partial fraction of 1/N(k-N). I have tried every method so far ( plotting roots, systems of equations). I think I found A=1/k but I have no clue how to find B value. I would really appreciate any help as I am a desperate student trying to pass the class. (the initial problem is dN/dt=(r/K)N(K-N), N(0)=No)

$\displaystyle \begin{align*} \frac{A}{N} + \frac{B}{K - N} &\equiv \frac{1}{N \left( K - N \right) } \\ A \left( K - N \right) + B \,N &\equiv 1 \end{align*}$

Let $\displaystyle \begin{align*} N = 0 \end{align*}$ to find $\displaystyle \begin{align*} A\,K = 1 \implies A = \frac{1}{K} \end{align*}$

Let $\displaystyle \begin{align*} N = K \end{align*}$ to find $\displaystyle \begin{align*} B \, K = 1 \implies B = \frac{1}{K} \end{align*}$

so the partial fraction decomposition is $\displaystyle \begin{align*} \frac{1}{K \, N } + \frac{1}{K \left( K - N \right) } \end{align*}$.
 
Although not asked for, it might be insightful for the OP to notice that the logistic equation is in fact a special case of the Bernoulli equation, and can be solved without partial fractions! However it would be advisable to ignore if you (the OP) have not yet covered this :).

The Bernoulli equation is of the form:

$\begin{equation}y' = f(x)y^a - g(x)y \end{equation}$

For convenience, let's jot down the logistic equation also,

$$x' = rx - rx^2$$.

Quite a resemblance, isn't there? Indeed, the Bernoulli equation with $a = 2$ and $f(x) = g(x) = -r$ is the logistic equation, also known as the Riccati equation.

Returning to the logistic equation and dividing through by $x^2$ gives,

$$x' = rx - rx^2 \implies x^{-2} x' = rx^{-1} - r$$.

Now we can make a (perhaps) devious substitution, $u = x^{-1}$.

It follows that, $u' = -x^{-2}x'$.

But we know what $x'$ is, so we can expand this differential equation in $u$,

$u' = -x^{-2}(rx - rx^2) = -r(x^{-1} - 1)$.

Thus, $u' + ru = r ... (1)$

Aha! The differential equation is now in it's linear form. We can use an integrating factor to finish off.

$$I(t) = e^{\int r dt} = e^{rt}.$$

Multiply (1) by $I(t)$,

$e^{rt}u' + e^{rt} ru = e^{rt} r$

We notice the following identity on the left hand side,

$\dfrac{d}{dt}(ue^{rt}) \equiv u'e^{rt} + rue^{rt}$, now we can integrate.$\int \frac{d}{dt}(ue^{rt}) dt = \int re^{rt} dt$

$ue^{rt} = e^{rt} + c$

$u = 1 + ce^{-rt}$

Recall that $u = x^{-1}$,

$x = \dfrac{1}{1 + ce^{-rt}}.$
 

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