Scott Pakin's Comprehensive LaTeX Symbol List identifies the structure of extensible delimiters in section **8.2 Resizing symbols** (p 100-102):

All variable-sized delimiters are defined (by the corresponding `.tfm`

file) in terms of up to five segments, as illustrated by FigureĀ 1 on pageĀ 102. The top, middle, and bottom segments are of a fixed size. The top-middle and middle-bottom segments (which are constrained to be the same character) are repeated as many times as necessary to achieve the desired height.

This is merely informative since I am unfamiliar with the procedure of how to create these things in a `.tfm`

file.

Conveniently, `scalerel`

provides `\scaleleftright[<max width>]{<left obj>}{<stuff>}{<right obj>}`

(and a comparable `\stretchleftright`

) for scaling/stretching both `<left obj>`

and `<right obj>`

to the height of `<stuff>`

(constrained, if required and optional, to a width of `<max width>`

). Here's a quick example:

```
\documentclass{article}
\usepackage{scalerel}% http://ctan.org/pkg/scalerel
\begin{document}
\[ \stretchleftright{\dagger}{\displaystyle\sum_{i=1}^n i}{\dagger} \]
\[ \scaleleftright{\dagger}{\displaystyle\sum_{i=1}^n i}{\dagger} \]
\end{document}
```

The glyph in question appears to be very similar to the uppercase L provided in the "curly" math font of the MathTime Professional 2 complete font set (commercial fonts).

It can be used in three ways:

## Method 1: change *all* math fonts to MTPro2

```
\documentclass{article}
\usepackage[mtpccal]{mtpro2}
\begin{document}
A set of strings is in the class $\mathcal{L}_{*}$ if and only if
$A$ is \dots within time $P(n)$, for some polynomial $P(n)$.
\end{document}
```

The package option `mtpccal`

assigns the "curly" script variant to `\mathcal`

. Depending on the font selections in your document, the other MTPro2 fonts may or may not be good matches with the body text (for example, it is a bad match with Computer Modern, as shown here). This also prevents use of the "standard" calligraphic fonts, if needed.

## Method 2: declare an additional symbol font (preserves existing settings for all other math)

```
\documentclass{article}
\DeclareSymbolFont{curly}{U}{mt2ms}{m}{n}
\DeclareSymbolFontAlphabet{\mathcurly}{curly}
\begin{document}
A set of strings is in the class $\mathcurly{L}_{*}$ if and only if
$A$ is \dots within time $P(n)$, for some polynomial $P(n)$.
\end{document}
```

This is more flexible than Method 1. You can make your own choices about math fonts and use the curly font only as needed. The disadvantage is that another full alphabet is used, which may or may not be an issue depending on the complexity of your document.

## Method 3: import this single symbol from MTPro2 only

Using the methodology from Importing a Single Symbol From a Different Font and borrowing the relevant code from `mtpro2.sty`

and `umt2ms.fd`

, this method brings in only the "L" from the MathTime Curly math font. It gives the same output as Method 2, but an additional alphabet is not used.

```
\documentclass{article}
\DeclareFontFamily{U}{mt2ms}{\skewchar\font42}
\DeclareFontShape{U}{mt2ms}{m}{n}{<-7>mt2mcf<7-9>mt2mcs<9->mt2mct}{}
\DeclareSymbolFont{MTPcurly}{U}{mt2ms}{m}{n}
\DeclareMathSymbol{\cobhamclass}{0}{MTPcurly}{'114}
\begin{document}
A set of strings is in the class $\cobhamclass_{*}$ if and only if
$A$ is \dots within time $P(n)$, for some polynomial $P(n)$.
\end{document}
```

## Best Answer

Without additional guidance, I made it based on

`\circ`

and defined it as`\mathbin`

.For a version that only works in the regular math size:

For the version that works across math styles:

Note that the final

`\circ`

in the definition of`\mathkey`

defines the vertical footprint of the`\mathkey`

symbol. Thus, one could change that to something else, let's say`X`

, in order to enlarge the resulting`\mathkey`

: