How can I grow the height of a function letter to match the height of the parameters?

Question

I'm only an occasional LaTeX user, and ever more rare math mode user, so maybe this is simple for others, but not for me:

I have a function named L*, and it has a fraction as parameter. In my first attempt ($L*\left(\frac{X}{X_N}\right)$) it looked as if I wanted to multiply L with the parameter, so I changed it to ${L*}\left(\frac{X}{X_N}\right)$.

Still in my eyes it looks like a multiplication (can I reduce the spacing between * and (?), so I want the size of L* match the height of the parameter, like this:

(As any TeX-eye can see I faked the size by using an image editor).

What I did try without success was a \left{L*}\frac{...}{...} which is not allowed, however.

Finally: When answering remember: Simple problems should have simple answers to be efficient. If the answer looks overly complicated (makes the formular very unreadable, I probably will not accept it.

Answer 1

I think it's a very bad idea.

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}

\newcommand{\Lstar}[1]{%
  \begingroup
  \sbox0{$L{*}$}%
  \sbox2{$#1$}%
  \ifdim\dimexpr\ht2+\dp2>1.5\dimexpr\ht0+\dp0\relax
    \vcenter{\hbox{\resizebox{!}{0.8\dimexpr\ht2+\dp2}{\box0}}}%
  \else
    L{*}%
  \fi
  #1
  \endgroup
}

\begin{document}

\[
\Lstar{(x)}+\Lstar{\left(\frac{X}{X_N}\right)}
\]

\end{document}

Answer 2

By default * is a binary operator so it has extra space put around it to make it look like multiplication. You can remove this space by enclosing it in braces: {*}. You could also use \ast instead of *, but this doesn't help because, as far as I can see, they are equivalent.

If you want to have a larger asterisk then you need to "leave" math-mode and change the font size. If you are going to do this often then I suggest using a macro like

\newcommand\Ast{\mbox{\large${\ast}$}}

Using these different options gives the following output:

From the OP, I suspect that the last option, which uses the macro above, is what you want. Here is the full code:

\documentclass{article}
\usepackage{amsmath}
\newcommand\Ast{\mbox{\large${\ast}$}}
\newcommand\Xn{\bigl(\frac{X}{X_N}\bigr)}

\begin{document}

  $L*\Xn$

  $L\ast\Xn$

  $L{*}\Xn$

  $L{\ast}\Xn$

  $L\Ast\Xn$

\end{document}

Btw, please always post a minimal working example with your questions so that people know what you are doing. This makes it easier for people to help you and decreases the chance that they will solve a "different problem" than what you are asking.