How to typeset an invisible paired delimiter, preferably using MathTools?

Question

I'm trying to typeset the simplification/expansion of a fraction using the notation below, but I would like the numerator to be centered and the 5\cdot 4) to be to the left and above of the entire fraction without the division line being distorted or stretched in any way by it.

I figured that if I could declare a set of invisible delimiters using \DeclarePairedDelimiter{<command>}{<left delimiter>}{<right delimiter>} the task would be simple, since I could simply place the fraction inside the delimiters and place the 5\cdot 4) relative to the entire block, but I'm not sure if this is possible. Using \DeclarePairedDelimiter{\invis}{.}{.} didn't work.

I'm open to other solutions, but if someone has a solution using this particular method, it would make my life a lot simpler.

Answer 1

I would like the numerator to be centered and the 5\cdot 4) to be to the left and above of the entire fraction without the division line being distorted or stretched in any way by it.

APPROACH 1

In this approach, the superscript will precede the full fraction, rather than just the numerator. It will also take up horizontal space, rather than be a lapped quantity.

EDITED to allow [l]eft or [r]ight positioning with use of optional argument.

\documentclass{article}
\usepackage{stackengine}
\newcommand\simpex[3][l]{%
  \savestack\thefrac{$\displaystyle#3$}
  \ifx l#1\rule{0pt}{\ht\thefraccontent}^{#2)\!}\fi
  #3
  \ifx r#1\rule{0pt}{\ht\thefraccontent}^{\!(#2}\fi
}
\begin{document}
\[
\simpex{5\cdot 4}{\frac{1}{17}}
\textrm{ versus }
\simpex[r]{5\cdot 4}{\frac{1}{17}}
\]
\end{document}

APPROACH 2

Being a \mathllap, there could be spacing issues with adjacent material. However, without more information on the use cases, it is hard to develop a remedy for that in the blind.

For example, will it always lead a line? Will it always be part of a fraction? If so, will it always be in the numerator? etc.

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[
\frac{\mathllap{^{5\cdot 4)}}1}{17}
\]
\end{document}

Answer 2

Is it one of these you want? They use stackengine:

\documentclass{article}
\usepackage{mathtools}
\usepackage[usestackEOL]{stackengine}
\setstackgap{S}{1pt}
\begin{document}

\[ A = \stackMath\Shortstack{\mathllap{\scriptstyle 5\cdot 4)}\\\dfrac{1}{17}} \]%

\[ A = \stackMath
\setstackgap{S}{-1ex}\Shortstack{\mathllap{\scriptstyle 5\cdot 4)\:}\\\dfrac{1}{17}} \]%

\end{document} 

Edit: This code uses only mathtools:

\[ A = \prescript{\mathllap{5\cdot 4)}\mkern-8mu}{}{\frac{1}{17}} \]%

Answer 3

Here's a solution that employs just the LaTeX macro \raisebox. The macro \mysimp is defined to take three arguments, one of which is optional. The optional argument sets the vertical displacement by which the first mandatory argument must be raised. The default value of the vertical displacement is 3ex, which seems about right if the final argument contains a display-style fractional term.

Remove the \scriptstyle directive if the left-superscript term should be typeset in text-style and not in \scriptstyle.

\documentclass{article}
\newcommand{\mysimp}[3][3ex]{\raisebox{#1}{$\scriptstyle #2$} \! #3}
\begin{document}
\[
\mysimp{5\cdot4)}{\frac{1}{17}} \qquad \mysimp[1.25ex]{uv}{abc}
\]
\end{document}

Answer 4

Here's a solution with only LaTeX macros (and \mathpalette).

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\aboveleft}[2]{{\mathpalette\above@left{{#1}{#2}}}}
\newcommand{\above@left}[2]{\above@@left{#1}#2}
\newcommand{\above@@left}[3]{%
  \sbox\z@{$\m@th#1#3$}%
  \raisebox{0.7\ht\z@}{$\m@th\dem@te{#1}#2$}%
  \box\z@
}
\newcommand{\dem@te}[1]{%
  \ifx#1\displaystyle\scriptstyle\else\scriptscriptstyle\fi
}
\makeatother

\begin{document}

\[
\aboveleft{5\cdot4)}{\frac{1}{17}}
\]
In text it is also possible $\aboveleft{5\cdot4)}{\frac{1}{17}}$

\end{document}