What is wrong with this LaTeX equation?

Question

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}

Answer 1

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. ;-)

Answer 2

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.