Let be a function on the positive integers defined by
Now let be defined as the number of times must be applied to for it to first equal 1.
For example, because and because .
How many solutions are there to the equation
We may derive an expression for g(x) by considering the decomposition of x into . In this way we may derive a recursively defined series that generates a value for any g(x) in terms of n and r. I have not carried out the full treatment, but have derived the following table:
I think you can guess how the pattern goes from this.
So to find g(5), for example, we do the following: . This is of the form where n = 2. So . Thus g(5) = 5 as expected.
We wish to find x such that g(x) = 20. We have several possibilities just in the small table I derived above:
So I've got g(1048576) = g(2056) = g(1027) = g(1026) as just some possibilities.
Once you derive the general relationship you should be able to just write x in terms of n and r.
(By the way, double check 4181; I'm getting g(4181) = 18.)