f(f(x)) = 4x

There are plenty of continuous functions {f} such that {f(f(x)) \equiv x}. Besides the trivial examples {f(x)=x} and {f(x)=-x}, one can take any equation {F(x,y)=0} that is symmetric in {x,y} and has a unique solution for one variable in terms of the other. For example: {x^3+y^3-1 =0 } leads to {f(x) = (1-x^3)^{1/3}}.

x^3+y^3 = 1

I can’t think of an explicit example that is also differentiable, but implicitly one can be defined by {x^3+y^3+x+y=1}, for example. In principle, this can be made explicit by solving the cubic equation for {x}, but I’d rather not.

x^3+y^3+x+y = 1

At the time of writing, I could not think of any diffeomorphism {f\colon \mathbb R \rightarrow \mathbb R} such that both {f} and {f^{-1}} have a nice explicit form. But Carl Feynman pointed out in a comment that the hyperbolic sine {f(x)= \sinh x = (e^x-e^{-x})/2} has the inverse {f^{-1}(x) = \log(x+\sqrt{x^2+1})} which certainly qualifies as nice and explicit.

Let’s change the problem to {f(f(x))=4x}. There are still two trivial, linear solutions: {f(x)=2x} and {f(x)=-2x}. Any other? The new equation imposes stronger constraints on {f}: for example, it implies

\displaystyle f(4x) = f(f(f(x)) = 4f(x)

But here is a reasonably simple nonlinear continuous example: define

\displaystyle f(x) = \begin{cases} 2^x,\quad & 1\le x\le 2 \\ 4\log_2 x,\quad &2\le x\le 4 \end{cases}

and extend to all {x} by {f(\pm 4x) = \pm 4f(x)}. The result looks like this, with the line {y=2x} drawn in red for comparison.

f(f(x)) = 4x

To check that this works, notice that {2^x} maps {[1,2]} to {[2,4]}, which the function {4\log_2 x} maps to {[4,8]}, and of course {4\log _2 2^x = 4x}.

From the plot, this function may appear to be differentiable for {x\ne 0}, but it is not. For example, at {x=2} the left derivative is {4\ln 2 \approx 2.8} while the right derivative is {2/\ln 2 \approx 2.9}.
This could be fixed by picking another building block instead of {2^x}, but not worth the effort. After all, the property {f(4x)=4f(x)} is inconsistent with differentiability at {0} as long as {f} is nonlinear.

The plots were made in Sage, with the function f define thus:

def f(x):
    if x == 0:
        return 0
    xa = abs(x)
    m = math.floor(math.log(xa, 2))
    if m % 2 == 0:
        return math.copysign(2**(m + xa/2**m), x)
        return math.copysign(2**(m+1) * (math.log(xa, 2)-m+1), x)

2 thoughts on “f(f(x)) = 4x”

  1. Forward and inverse hyperbolic sine both can be expressed in an explicit form, taught in high school. Is that close enough to a “nice” invertible diffeomorphism?

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