In the math mode, I am typing an equation and I am constantly getting errors such as
Undefined Control Sequence etc.
I have tried all things to my knowledge, but I am missing something. Can someone help me?
\begin{equation}
\large \alpha_b = \left\{
{\text{1, I_{i,r}^b \geq I_{thr}} \atop
\text{0, I_{i,r}^b < I_{thr}}
\right}
\end{equation}
This is the correct way to type in the equation.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\alpha_b =
\begin{cases}
1, & I_{i,r}^b \geq I_{thr} \\
0, & I_{i,r}^b < I_{thr}
\end{cases}
\end{equation}
\end{document}
Depending on its meaning, “thr” could be better as
I_{\mathrm{thr}}
Use this if it's a textual abbreviation.
About your code, it's hard to find something right, I'm afraid.
The command \large is not allowed in math mode. Probably you wanted to use it for enlarging the result of \atop. But \atop is not to be used anywhere in LaTeX, if you load amsmath (and you do, because you have \text).
The command \text is used to insert textual parts in an equation, but you're using it for math parts; finally \right} is not allowed and it should be \right. for a null delimiter.
However, using a fraction for doing a cases environment is wrong to begin with. I'm quite surprised you know about \atop and not about cases. If \atop has been suggested to you by a friend, this could be a good occasion for breaking this friendship. ;-)
\\ :P :)
– Au101
Nov 9 '16 at 12:37
mathtools instead of amsmath. (I know it doesn't make a difference here.) Also, what's the difference between \mathrm and \textup in the subscript?
– Svend Tveskæg
Nov 13 '16 at 20:16
mathtools should be rewritten from scratch, so I load it only when it's really necessary.
– egreg
Nov 13 '16 at 20:18
Here is a possible solution:
\documentclass{article}
\begin{document}
\begin{equation}
\alpha_b
= \left\{\frac{1, I_{i,r}^b \geq I_{thr}}{0, I_{i,r}^b < I_{thr}}\right\}
\end{equation}
or
\begin{equation}
\alpha_b
= \left\{\frac{1, I_{i,r}^b \geq I_{\mathrm{thr}}}{0, I_{i,r}^b < I_{\mathrm{thr}}}\right\}
\end{equation}
\end{document}
The latter should be used if the subscripts to the right are text.
_is a math mode command. It must only appear in math mode. You have it within\text{ ... }which puts you in text mode unsurprisingly. So_is undefined and fails – Au101 Nov 9 '16 at 3:53\right}at the end, where I assume you want\right\}– Au101 Nov 9 '16 at 3:58