How do I make my nicefrac fraction line larger?

Question

I am trying to replicate this formula with LaTeX:

However, I am struggling to make the fraction look the same. This is my attempt:

\begin{equation}

\label{eq:alphaK}

\alpha(k) = \frac{1}{(N-k)} \nicefrac{ \left( \displaystyle \sum_{i=k+1}^{N} \lambda_i \right) }{\left( \displaystyle \prod_{i=k+1}^N \lambda_i \right)^{\frac{1}{N-k}}}

\end{equation}

Which results in the following:

I am looking for a way to increase the size of the "slash" (/) that separates the numerator and denominator. I know I could use a normal \frac instead of \nicefrac but I like how it looks when num/den are displayed side by side.

Is there any way I can force the fraction to look more like the first pictured one?

Answer 1

There are many ways. You will find two different in some variation below. The last one looks more like your desired output.

\documentclass{memoir}
\usepackage{amsmath,xfrac,mathtools}

\begin{document}
\begin{align}
    \alpha(k) 
        &=\frac{1}{(N-k)} 
            \left.
                \left(\sum_{i=k+1}^{N} \lambda_i \right)
            \right/
            \left(\prod_{i=k+1}^N \lambda_i \right)^{\sfrac 1{(N-k)}}\label{eq:alphaK}\\
        &=\frac{1}{N-k} 
            \left.
                \left(\sum_{i=k+1}^{N} \lambda_i \right)
            \middle/
                \left(\prod_{i=k+1}^N \lambda_i \right)^{\frac 1{N-k}}
            \right.\\
        &=\frac{1}{N-k} 
            \biggl(\,\sum_{i=k+1}^{N} \lambda_i \biggr) 
            \bigg/ 
            \biggl(\,\prod_{i=k+1}^N \lambda_i \biggr)^{1/{(N-k)}}\\
            %space added as suggested by Enrico Gregorio
        &=\frac
            {\sum\limits_{i=k+1}^{N} \lambda_i}
            {(N-k)\, \sqrt[N-k]{\prod\limits_{i=k+1}^N\lambda_i}} 
        &\geq 1 \textup{ (by the AM–GM inequality)}
\end{align}
\end{document}

---

Remarks.

• Please pay attention to Zarko's comment.

• The nicefrac package might be replaced by https://ctan.org/pkg/xfrac.

• The question mathtools brackets larger than \Bigg might be of interest for you.

• Another useful package is https://ctan.org/pkg/mathtools, look for the sections dealing with delimiters.

• Observe that I changed some aspects of the formula, which might collide with your notation.

• The video https://youtu.be/LFrdqQZ8FFc might also be helpful.

Answer 2

a small variation of CampanIgnis answer (since he was to fast for my frozen fingers :-( ):

\documentclass{article}
\usepackage{nccmath}

\begin{document}
\begin{equation}%\label{eq:alphaK}
\alpha(k) = \frac{1}{(N-k)}
            \left(\sum_{i=k+1}^{N} \lambda_i \middle)
            \!\middle/\!
            \middle(\prod_{i=k+1}^N \lambda_i \right)^{\mfrac{1}{N-k}}
\end{equation}
\end{document}