Why \operatorname Does Not Render Polish Letters in LaTeX

The letters ł and Ł are text characters with diacritics, not ordinary symbols in LaTeX’s standard math alphabets. As a result, \operatorname{ł} may fail or produce a missing glyph, even when the same character works in ordinary text.

Use \text{...} from amsmath to typeset the letter with the document’s text font. If it should act as a named operator—with operator spacing—put that text inside \mathop.

Why \operatorname can fail

\operatorname{...} is intended for operator names set in a math alphabet, typically names made from ordinary Latin letters, such as \operatorname{rank}. It does not make every Unicode character available in math mode. Polish letters may be absent from the math font or encoding in use.

Text and math fonts are separate. A document can therefore display ł in a paragraph but fail to display it inside \operatorname{...}. The fix is to use a text font for the character rather than asking a math alphabet to provide it.

A minimal working example

This example uses pdfLaTeX. T1 encoding provides better support for accented characters in text fonts, and the Polish language option sets suitable language conventions. The selected text font must include the Polish glyphs.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[polish]{babel}
\usepackage{amsmath}

\newcommand{\lop}{\mathop{\text{ł}}\nolimits}
\newcommand{\Lop}{\mathop{\text{Ł}}\nolimits}

\begin{document}

The letters in text: ł and Ł.

In math, as operator names:
\[
  \lop(x) + \Lop(x)
\]

As ordinary text within a formula:
\[
  x_{\text{ł}} = 1
\]

\end{document}

The \text command comes from amsmath (through its amstext support) and uses the surrounding text font at the appropriate size for the math style. The \mathop command gives the text operator spacing; \nolimits puts any limits beside the operator, as is usual for named functions. If you want limits above and below in display math, use \limits instead.

For a one-off operator, you can write \mathop{\text{ł}}\nolimits directly. If the letter is only a label or subscript—not an operator—use \text{ł} without \mathop.

Engine and font considerations

With pdfLaTeX, \usepackage[T1]{fontenc} is often the key addition when a Polish character is missing or poorly encoded. The chosen text font must also contain the glyph. Modern LaTeX uses UTF-8 input by default; the inputenc line in the example is harmless in current installations and can be omitted there.

With LuaLaTeX or XeLaTeX, input is Unicode by default. You can use a Unicode text font that contains ł and Ł, commonly selected with fontspec. The same principle still applies: use \text{...} for the character inside a formula rather than relying on a math alphabet to contain it.

Common mistakes

• Keeping the character directly in \operatorname{...}: this still asks the math operator font to supply the glyph.
• Using \mathrm{ł} as a universal fix: \mathrm selects a math alphabet, so it may have the same missing-glyph problem.
• Assuming UTF-8 alone solves font coverage: UTF-8 handles the input character; it does not guarantee that the selected font contains it.
• Using \text{ł} when operator spacing is needed: the letter will print, but it will behave as an ordinary text box. Add \mathop{...}\nolimits to give it operator spacing.