LuaLaTeX invisible unicode characters

Question

I have a minimal document:

\documentclass{article}
\begin{document}
this is λ lambda
\end{document}

When building with latex, is gives the error

Package inputenc: Unicode character λ (U+03BB)
(inputenc)    not set up for use with LaTeX.

and when building with lualatex, it outputs:

(the unicode λ character is invisible)

How to make the λ visible?

Answer 1

It's not enough to provide utf8-encoded input. You also need to select text and math fonts that possess the glyphs you wish to typeset.

As the following screenshot reveals, Latin Modern Roman (the default system serif text font) doesn't feature a λ glyph, but Latin Modern Math does -- in both a slanted and in an upright shape. Stix Two, in contrast, possesses a λ glyph in both text and math.

\documentclass{article}
\usepackage{unicode-math} % loads 'fontspec' automatically
\begin{document}
\obeylines % just for this example
\setmainfont{Latin Modern Roman}
\setmathfont{Latin Modern Math}
Latin Modern text and math font
this is λ lambda
this is $λ$ $\symup{λ}$ lambda
\bigskip
\setmainfont{Stix Two Text}
\setmathfont{Stix Two Math}
Stix Two text and math font
this is λ lambda
this is $λ$ $\symup{λ}$ lambda
\end{document}

Answer 2

You need to load both a font and an encoding that supports Greek. In LuaLaTeX, that’s Unicode, and you can automatically switch to your Greek language and font whenever you start typing in Greek.

\documentclass{article}
\tracinglostchars=2
\usepackage[english]{babel}
\usepackage{unicode-math}
\babelprovide[import=el, onchar=ids fonts]{greek}
\babelfont{rm}[ Language=Default]{NewComputerModernBook}
\babelfont{sf}[ Language=Default]{NewComputerModernSansBook}
\babelfont{tt}[Language=Default]{NewComputerModernMonoBook}
\setmathfont{NewCMMath-Book}
\pagestyle{empty} % Suppress page numbering
\begin{document}
This is λ lambda.
And (λ) in math mode.
\end{document}

You can define different fonts for \babelfont[greek]{rm}, and Babel will switch to them when you start typing in Greek in text mode. It will use the symbols from your Unicode math font in math mode. You can instead load Polyglossia if you prefer it to Babel.

My advice is to use LuaLaTeX and unicode-math when you can, and legacy 8-bit fonts when you have to. Tn PDFTeX, the 8-bit font encoding you want is called LGR, and you also want alphabeta to enable Greek letters in both text and math mode.

\documentclass{article}
\tracinglostchars=2
\usepackage[LGR,T1]{fontenc}
\usepackage[greek, english]{babel}
\usepackage{alphabeta}
\pagestyle{empty} % Suppress page numbering
\begin{document}
This is λ lambda.
And (λ) in math mode.
\end{document}

If your main font does not come in LGR, you can declare a font substitution with substitutefont.

You were getting the error you did because PDFTeX only understands Unicode characters that you’ve set up, and the LaTeX kernel only sets up characters that are in one of the font encodings you’ve loaded.

If you check your .log file from your original LuaLaTeX MWE, you’ll see a warning message buried in the middle of it, saying that there was no λ glyph in the current font. Other than that, TeX silently ignores the error. This is terrible design, but apparently we’re stuck with it for the sake of backward compatibility. The command \tracinglostchars=2 will at least display the warning on the console.

Answer 3

Mico's answer is correct but he doesn't give an example font to use which has Greek letters so I wanted to add the font GFS Artemisia by the Greek Font Society works well. The following should work

\documentclass{article}
\usepackage{unicode-math} % loads 'fontspec' automatically
\setmainfont{GFS Artemisia}
\begin{document}
this is λ lambda
\end{document}