# Moving to imaginary time

Before moving to imaginary time let us warm ourselves up first doing a small exercise while still in real time. We did not quite finish our business with periods of elliptic functions in the last post “Periods of Jacobi elliptic functions – Part 1.” We did not answer the question: what about the periods for ?

We know (see The case of inverted modulus – Treading on Tiger’s tail and Jacobi elliptic cn and dn) that for we have the formulas

(1) We know, from Periods of Jacobi elliptic functions – Part 1, that the period of for is Therefore, from Eq. (1) we deduce that the period of for is For instance, we have But in the previous post we have shown that for we have

(2) Therefore, for the period of is From two last equations in (1) we see that, for , the period for is the same as that of , and the period of is half of that.
Below is the plot of the function that shows its periodicity. ##### Plot of for and  After these warm-up exercises we now open the door to the universe with the imaginary time. In fact there are several different doors leading there, and we will choose one that has been used by Alfred Cardew Dixon, MA, in his 1894 book “The Elementary Properties of the Elliptic Functions with Examples“.

Namely, we take the Eqs. (1)-(4) from Periods of Jacobi elliptic functions – Part 1 as the defining relations of the Jacobi elliptic functions.
In other words: whenever we have three functions and a constant satisfying

(3) (4) (5) (6) then

(7) We are now ready for the trick invented by Jacobi, called “Jacobi Imaginary Transformation“. But before that, once we are in the land of elliptic functions, let us familiarize ourselves with the language used by some of the native population of this country. Following one Dr. Glaisher, the following notation is sometimes used in conversations in local pubs:   It is a question of simple rules about derivatives of fractions and a simple algebra (we can use REDUCE to this end) to establish the following properties:

(8) (9) (10) Here is the REDUCE code checking these relations – it produces, on output, three zeros.:

OPERATOR cn,sn,dn,sc,dc,nc;
For all u let sc(u)=sn(u)/cn(u);
For all u let dc(u)=dn(u)/cn(u);
For all u let nc(u)=1/cn(u);
For all u let DF(sn(u),u)=cn(u)*dn(u);
For all v let DF(cn(v),v)=-sn(v)*dn(v);
For all v let DF(dn(v),v)=-k2*sn(v)*cn(v);
For all u let cn(u)^2=1-sn(u)^2;
For all u let dn(u)^2=1-k2*sn(u)^2;
kp2:=1-k2;
l1:=DF(sc(u),u)-dc(u)*nc(u);
l2:=nc(u)^2-sc(u)^2-1;
l3:=dc(u)^2-kp2*sc(u)^2-1;
END;

Comparing Eqs. (810) with (35) we notice that if we set ,
then, since we must have Therefore   Bu since the relation between and is symmetric, we can as well write

(11) The functions on the right hand side have the period denoted simply as . Therefore the function on the left hand side have the period We can use now addition formula and
calculate the functions for any complex The period along the real direction is the period along the imaginary direction is  ##### Plot of of the absolute value of for We have , Now we can navigate with ease in our complex times. In real time and in imaginary time. ## 15 thoughts on “Moving to imaginary time”

1. arkajad says: Real time behind my window now

2. arkajad says:

@Bjab
Thanks for errata. Fixed!

3. Bjab says:

Which of those two-letter-functions got real values, which imaginary, which complex?

1. arkajad says:

For real argument they are all real. For imaginary argument sn is imaginary, cn and dn are real. So, whenever the two letter function has s, it is imaginary, otherwise it is real.
For a complex argument they will all be, in general, complex.

4. Bjab says:

“We have 4K=4K(0.8)=9.02882, 4K’=4K(0.2)=6.63849.”

I don’t recognize that values on the graph.

1. arkajad says:

Since we are plotting the absolute value, the period on the graph will be half-period: 4.5 for real, 3.3 for imaginary, and these we can see.

1. Bjab says:

Ark,
cn(z) is a real valued function, I suppose.
So please present the graph not of absolute value of this function but the value itself.
(Graph with the same domain and m as in the post).

1. arkajad says:

For complex z, cn(z) is, in general, complex. It is real for imaginary z.

2. Bjab says:

“We can use now addition formula and
calculate the functions … “

Addition formula – what is it?

1. arkajad says:

Formula (5) in Periods of Jacobi elliptic functions – Part 1

1. Bjab says:

So formula (5) also relates to not real arguments?
I didn’t think that way.

2. Bjab says:

It looks like something here eats its own tail.

3. arkajad says:

Perhaps, but it will not bite you. The main points are:

1) how to define these functions for complex z.
2) How to prove the addition formulas for complex z

Here is the possible answer: we use the addition formulas to define the functions for a complex argument. Then we verify that they still hold for a general complex argument.

5. Bjab says:

So what about graphs of real and imaginary parts of cn(z) ?

6. arkajad says:

@Bjab Real part. Imaginary part. Two parts together.