How to use lambda(λ) symbol in LaTeX?

You do not need to use any package to write the lambda symbol in LaTeX, just use the default \lambda command and the symbol will be printed in the document.

\documentclass{article}
\begin{document}
   \[ 1 = \lambda f.\lambda x. x \]
   \[ \lambda(x-a) \]
   \[ \lambda\left(\frac{1}{x}\right) \]
\end{document}

Output :

lambda-bar(ƛ) in LaTeX

Sometimes we need the lambda-bar symbol in the equation, in that case, you need to use a package called txfonts. You can print this symbol in a LaTeX document with the \lambdabar command included in this package.

\documentclass{article}
\usepackage{txfonts}
\begin{document}
  \[ \verb|\lambda|\rightarrow \lambda \]
  \[ \verb|\lambdabar|\rightarrow \lambdabar \]
\end{document}

Output :

Bold lambda and lambda-bar symbol

You need to use the bm package to write the lambda and lambda-bar symbols in bold font. The \bm command included in this package allows you to print these symbols in bold in a LaTeX document. take a look

\documentclass{article}
\usepackage{txfonts}
\usepackage{bm}
\begin{document}
  \[ \verb|\lambda|\rightarrow \lambda \quad \verb|\bm{\lambda}|\rightarrow \bm{\lambda} \]
  \[ \verb|\lambdabar|\rightarrow \lambdabar \quad \verb|\bm{\lambdabar}|\rightarrow \bm{\lambdabar} \]
\end{document}

Output :

• Big Lambda(Λ) symbol in LaTeX