Posted by: **Dave Richeson** | March 9, 2009

## Sines and cosines (part 1)

A friend asked me the following question. What -values satisfy the equation

?

It occurred to me to ask a more general question. Let and . Let be the composition of with itself times. Similarly, let be the composition of with itself times.

What are the solutions to ?

Is it possible answer this question? Always? Sometimes? Never?

I’ll post my thoughts on this in a follow-up post.

[Update: I forgot to add that when I was looking up this problem online I found that it was a problem in the *1994 Russian* Math *Olympiad.]*

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This was a great question. I had fun working on it. I’m a little sad that you already posted your solution – I wanted to have a crack at working on it and posting how far I got. I was going in a similar direction as you, and I even was using the same Grapher program on the Mac to generate my curves!

I too found that as m and n increase, we tend to get something that approximates a line. I like the cobweb plots. I don’t know why I didn’t think of them immediately, since this is precisely what they’re for!

I might make this into a math club investigation for next year.

By:

samjshahon March 9, 2009at 11:38 pm

Sorry to post the solution too quickly. I posted the solution separately so readers could read the question and the solution separately, but maybe a time lag would have been better—remove all temptation!

By:

Dave Richesonon March 10, 2009at 12:02 am

A numerical solution is 0.756887 + 0.610155 i, but I don’t know a good way to find or express it analytically.

By:

Nathanon April 9, 2009at 8:40 pm