undefined control sequence in latex

Question

I snippet of my code is presented here.I do not understand why I'm getting the error undefined control sequence.I have also included the package amsmath.

\subsection{Logistic Regression}
\noindent $p_i=P(y_i=1)$ and $(x_i\in\mathbb{R})^d$. The binary responses are modelled using the following formulation $$\log{{p_{i}}\over{1-p_{i}}}={{\beta}}^{T}{\bf x}_{i}\qquad\text{or}\qquadp_{i}={{\exp({{\beta}}^{T}{\bf x}_{i})}\over{1+\exp({{\beta}}^{T}{\bf x}_{i})}}$$ where $\beta\in\mathbb{R}^d$ are some unknown regression coefficients often estimated using maximum likelihood. 

Answer 1

To use \mathbb, you also need to add amssymb:

In this instance, there seems to be no need for \noindent (unless you modified your sectional headings from the default). Note that I've completely rewritten the mathematical expression:

• Don't use $$...$$ for your math display; rather use \[...\]. See Why is \[ … \] preferable to $$ … $$?;

• Don't use {<num>}\over{<denom>} for a fraction. Instead, use \frac{<num>}{<denom>};

• Don't over-use braces; they could change the spacing of elements in the expression.

Answer 2

Here is a polished version of your snippet:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb} % for \mathbb

\begin{document}

\subsection{Logistic Regression}

$p_i=P(y_i=1)$ and $(\mathbf{x}_i\in\mathbb{R}^d)$. The binary responses are 
modelled using the following formulation
\[
\log \frac{p_{i}}{1-p_{i}}=\beta^{T}\mathbf{x}_{i}
\qquad\text{or}\qquad
p_{i}=\frac{\exp(\beta^{T}\mathbf{x}_{i})}{1+\exp(\beta^{T}\mathbf{x}_{i})}
\]
where $\beta\in\mathbb{R}^d$ are some unknown regression coefficients often 
estimated using maximum likelihood.

\end{document}