Yesterday, in “The case of the swinging pendulum“, I started my writing about **nonlinear mathematical pendulum**, and how it can be solved using **Jacobi elliptic functions**. I did not finish. My idea was to follow the derivation in the book by Lawden. But then I slept over it and this morning I decided to do it differently, I believe in a better way. Or, at least, to do it the way that gives me a satisfaction of being “more elegant”. But in order to do it I had to return to pure math and derive some properties of the derivatives of our functions. So, here it is **pure math. We will be using these properties later on**.

Differential equations satisfied by Jacobi elliptic functions am, sn, cn, dn.

**We will first deal with the case when the modulus**

We start with the definition of the incomplete elliptic integral of the first kind defined in Eq. (1) of Jacobi amplitude- realism or cubism:

(1)

The derivative of with respect is then simply the integrand function:

The amplitude function is the inverse function of Therefore the derivative of with respect to is the the inverse of

But, from the definition of the function , Therefore

Now, using the definition of the function (Eq. (5) in Derivatives of Jacobi elliptic am, sn, cn, dn) we get

We can now calculate the derivative The function is a composed function: Therefore

From the definition of we know that therefore using Eq. (2) we obtain

Much the same way we get

To calculate we use the definition

or

We will now derive differential equation satisfied by the function We have

But therefore

**This last equation will be just right for solving the pendulum problem,**

The formulas above were derived assuming It can be now verified, in a quite straightforward way, that** they are valid also for **

I am not posting the derivation here – they follow simply from the definitions and from the formulas above. The simple calculations can be downloaded form here: jacobi_cn_and_dn.pdf.

Erratum

I thnink that whenever there is k as an argument of a function it should be replaced by m (in six places).

the inverse function of F(u,\phi). ->

the inverse function of F(\phi,m).

->

@BJAB Thanks for the errata. Fixed twice.