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
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\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.