# Thread: Showing that derivative tends to infinity (Poisson solution)

1. ## Showing that derivative tends to infinity (Poisson solution)

Hi,

In the plot below I have $f(\theta) = sin(\theta)$.

The second graph of the plot shows that, for x = 1, the plot of $P(\theta, x) = f(\theta - x f(\theta))$ has an infinite slope at $\theta = 0$ (called the Poisson non-linear propagation solution).

I'm trying to show this analytically but not getting the right answer. I reproduce my steps below.

We want to show that for $x = 1$, $P(\theta = 0, x)$ has an infinite slope, i.e. $dP/d\theta \rightarrow \inf$.

$P(\theta, x) = f(\theta - x f(\theta))$

we have $f(\theta) = sin(\theta)$, so

$P(\theta, x) = sin(\theta - x sin(\theta))$

$dP/d\theta = cos(\theta - sin(\theta))(1 - x cos(\theta))$

What I'm trying to show is $dP/d\theta \rightarrow \inf$, but that's clearly not possible from my result above.

Keen to hear where I'm going wrong. Thanks

2. ## Re: Showing that derivative tends to infinity (Poisson solution)

Use the definition of a derivative at a point x=a. where the derivative of some function at a specified point is lim x->a (f(x)-f(a))/(x-a) to solve your problem

-Dan

4. ## Re: Showing that derivative tends to infinity (Poisson solution)

@ShadowKnight8702: I'm doing a similar sort of thing but not getting the right answer. From post #1 I get an answer that won't tend to infinity under any circumstances.

@topsquark: The problem is more concisely put in this thread.