# Undefined control sequence $\Lambda \in \mathb distributions, where $\Lambda \in \mathb{Z}^{+}$ is a security par... Please guide me to overcome this error Thanks • try with \mathbb{Z} – touhami Jan 20 '16 at 17:38 ## 1 Answer 1. There is a typo (\mathbb is the correct command). 2. You must have the amsfonts package added in the preamble. This example works: \documentclass{article} \usepackage{amsfonts} \begin{document}$\Lambda \in \mathbb{Z}^{+}$\end{document}  UPDATE. The compiler reports the command it did not recognise. If you are running pdflatex or whatever from the command prompt, you are likely to have seen this message: ! Undefined control sequence. l.4$\Lambda \in \mathb
{Z}^{+}\$


Look at the second line. l.4 means that the unexpected thing is in the fourth line of the input file, and the last thing in the line before the break is the culprit (in this case, \mathb). Please examine the error message: if there is a typo, it will always say “Undefined control sequence” and then put the unrecognised command before the break. Please look at that and try to figure it out for yourself at first.