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An involution is a function $f:X\to X$ such that $f\circ f=\text{id}$. Is there a name for a function $g:X\to X$ such that $f\equiv g\circ g$ is an involution? An example is multiplication by $\pm i$ in the complex plane (or more generally for an algebra over $\mathbb{C}$, or for some space which is a product with $\mathbb{C}$). In an almost complex structure, the linear map $J$ with $J^2=-\text{id}$ is also an example. Yet another example is a (properly normalized) Fourier transform, where squaring the Fourier transform $\mathscr{F}$ gives the involution $\mathscr{F}^2[f(t)]=f(-t)$ for square-integrable functions.

In a group, generally, this is clearly a 4-cycle. But considering the connection with complex and almost-complex structures (and quaternions, which have multiplication by $\pm i,\pm j, \pm k$ as 4-cycles) I thought there may be a special name for such a function on a more general space.

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I'm afraid I've only always used "of order (dividing) four" for those. –  Hagen von Eitzen Jun 25 '13 at 20:46
    
Involulution. (random characters for my answer to be submittable) –  oxeimon Jun 26 '13 at 6:46

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I've never heard a special word for this. Things whose $n$th power is $1$ are usually just called $n$th roots of unity, but perhaps someone employed a special name in some context.


I'm sorely tempted to call it a "spinvolution" because of your two examples. In mathematical physics, there are things (most everything we interact with) that are invariant under a rotation of $2\pi$, and then there are other quantities called spinorial quantities which transform to their negative under a rotation of $2\pi$. Both of your examples really lend themselves to this "spinor" picture :)

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