# Math symbol doesn't work properly

Something quite strange. Why I have as a result of the following formula:

$$\omega=\lambda_{1}\Sigma^{-1}\bf{1}+\lambda_{2}\Sigma^{-1}\mu$$


a black square instead the sigma symbol? in the first part is ok. I am becoming a bit crazy trying to find out the error. I tried everything, to separate, to isolate, to write the formula with double  rather than , packages loaded are the normal ones, etc... Any other symbol works perfectly but with \Sigma is quite weird, this is what I got no matter the way you try to write the formula.

• \bf should never be used in latex but if you do use it the syntax would be {\bf 1} but use \mathbf{1} – David Carlisle Jul 23 '18 at 11:20
• Your usage of \bf{} is wrong, it should be \mathbf{}. A long time ago in a galaxy far far away it should have been {\bf }, but not any more. I suspect what's happened is now everything after \bf is in the bold font and \Sigma's place in the bold font encoding is taken by the square – Au101 Jul 23 '18 at 11:22
• You're right. My fault. Sorry for the inconvenience :( – José Jul 23 '18 at 14:43
• @José No need to apologise - not an inconvenience - it's always good to be able to correct these errors which are easy to make :) – Au101 Jul 23 '18 at 15:31

$$\omega=\lambda_{1}\Sigma^{-1} \mathbf{1}+\lambda_{2}\Sigma^{-1}\mu$$

Only the \bf command was exchanged with \mathbf.