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 $f(x)=2+\sin x$.
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] *)
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.
$\endgroup$
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
