How do I keep the Standard Form from reducing negative powers?

Question

I have the following input form:

R[η_] := (3*Subscript[ρ, B]*aEta[η]^-3)/(4*Subscript[ρ, γ]*aEta[η]^-4)

It's supposed to look like this: $$R[\eta\_]:=\frac{3 \rho_B\space a_{\eta}[\eta]^{-3}}{4\space\rho_{\gamma}\space a_{\eta}[\eta]^{-4}}$$ But when I convert it to Normal Form, Mathematica attempts to simplify the negative powers and I get: $$R[\eta\_]:= \frac{3\rho_B}{\frac{a_{\eta}[\eta]^3(4\rho_{\gamma})}{a_{\eta}[\eta]^4}}$$ Is there anyway to force the Standard Form to show the way it's supposed to look (i.e. the way the formula is traditionally written in the textbooks). Perhaps some association operator that can't be overridden when converting from Input to Standard?

Answer 1

Sometimes I forget the fact that

FullForm[x^-2] === FullForm[1/x^2] (* True *)

Borrowing a trick from another post, perhaps this will work for you:

xPower /: MakeBoxes[xPower[x_, e_ /; e < 0], form_] := SuperscriptBox[MakeBoxes[x, form], MakeBoxes[e, form]] 
xPower := Power 
R[\[Eta]_] := HoldForm[Times[3, Subscript[\[Rho], B], xPower[Subscript[a,\[Eta]][\[Eta]],-3]]/Times[4,Subscript[\[Rho],\[Gamma]],xPower[Subscript[a,\[Eta]][\[Eta]],-4]]]
R[\[Pi]] // StandardForm

I can't think of a solution that will let you keep using those shortcuts mentioned in the comments. Sometimes you just have to separate the display of a symbol from its function.

Also, on macOS, the shortcuts for converting a selection to StandardForm or InputForm is Command + Shift + N and Command + Shift + I respectively.