I want to be able to write a specific variation of the letter λ (\lambda) without altering any other letters or symbols

Question

This lambda is from the txfonts package, but I don't like the other symbols in that package and I only want to use this specific symbol. How do I do this?

Answer 1

Prepare a simple file:

\documentclass{article}
\usepackage{txfonts}
\show\lambda
\stop

Upon running pdflatex on this you'd get the message

> \lambda=\mathchar"115.

This tells you that \lambda is taken from the font in mathgroup 1 at the slot "15 (hexadecimal).

OK, now modify \show\lambda into

$ $\showthe\textfont1

and run again pdflatex to get

> \OML/txmi/m/it/10 .

The empty math formula sets up the math fonts and we can inquire about the font in mathgroup 1.

Good! You have all the needed information. We can now import the single symbol using \text for being sure it changes size in sub/superscripts.

\documentclass{article}
\usepackage{amsmath}
\DeclareRobustCommand{\varlambda}{\text{\usefont{OML}{txmi}{m}{it}\symbol{"15}}}
\begin{document}
$\alpha\beta\lambda\varlambda$
\end{document}