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I want to define an operator $G$ such that

$$G(f):=\begin{cases}f(\{x\}),&\lfloor x\rfloor\text{ is even}\\\frac1{f(\{x\})},&\lfloor x\rfloor\text{ is odd}\end{cases}$$

for any function $f$, where $\{x\}$ means "fractional part of $x$". I dont have a clue about how to do this. I wanted to write something like

G[2 + Sin[x]]

that define the above over the function $f(x)=2+\sin x$.

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g[f_][x_?NumericQ] := If[EvenQ@Floor[x], f@FractionalPart[x], 1/f@FractionalPart[x]]

The ?NumericQ part is important because EvenQ returns False immediately for anything that is not a number.

In this case, f needs to be an actual function. Sin[x] and Sin[x]+2 are not functions. They are expressions in terms of x. Sin and Sin[#]+2& are functions. Look up Function to see what # and & mean.

Example:

g[Sin][5/2]
(* Sin[1/2] *)
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  • $\begingroup$ I know how to build pure functions. Thank you very much. $\endgroup$ – Masacroso May 10 '17 at 13:32
  • $\begingroup$ @Masacroso Great! I was not sure, so I mentioned it just in case. In your post you show an expression like 2+Sin[x] instead of a function. If you have an expression like that, normally you also need to specify the variable, x. That's how Plot, Integrate, etc. work. $\endgroup$ – Szabolcs May 10 '17 at 13:48
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As an alternative to Szabolcs' answer I would sugest a different approach, that quite literally follows the mathematical definition of $G(f)$. Note that we may avoid the premature evaluation of any argument that is not numeric by using Divisible instead of EvenQ or OddQ:

g[ f_Function ] := Function[ x, 
    Piecewise[
        {
          {     f[ FractionalPart @ x ],       Divisible[ Floor[x], 2 ] },
          { 1 / f[ FractionalPart @ x ], Not @ Divisible[ Floor[x], 2 ] }
        },
        Indeterminate (* in all other cases *)
    ]
]

We may then use this for numeric arguments:

f = Function[ x, 2 + Sin[x] ];
h = g[f]; (* or directly g[f] @ x *)
h[ 5/2 ]

$2 + \text{Sin}[\frac{1}{2}]$

In the given form we can now also work symbolically:

h[x] // Head

Piecewise

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