Equations with several identities

Question

I want to write a simple single line equation of this kind: a = b + c = d + e + f. However, when I try to add the second identity ( = ), everything on its right side does not appear in the results. Following a piece of code here I used the &= for either the second identity or both the first and the second. Still I am doing something wrong. Here follows the code I am trying to use

\begin{equation}
\hat{{\varphi}_{it} (i_{it}, k_{it})} &= \hat{y_{it}} - \hat{{\beta}_{l}} l_{it} - \hat{{\vartheta}_{1}} X_{it} - \hat{{\vartheta}_{2}} Z_{it} &= \hat{{delta_{0}} + \sum{\sum{{\delta}_{ij} k_{t}^{i} i_{t}^{j}}} - \hat{{\beta}_{l}}l_{it} - \hat{{\vartheta}_{1}} X_{it} - \hat{{\vartheta}_{2}} Z_{it}
\end{equation}

I get the 3 following error messages exactly, attributed to the line after the command \end{equation}:

 Paragraph ended before \mathaccent\/ was complete
 Missing $ inserted
 Display math should end with $$

Answer 1

Is this what you try to obtain? The first approach is a pure/simple math environment, the second is use of array environment, and the third approach is use of align environment. This solution simplifies the code.

Note: There was an extra \hat on the second equation right after = without an ending } and was removed. Also a backslash was missing for delta in the second equation.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

% -- case 1

\begin{equation}
%\begin{array}{lll}
\hat{\varphi}_{it} (i_{it}, k_{it}) =\hat{y}_{it} - \hat{\beta_l} l_{it} - \hat{\vartheta}_1 X_{it} - \hat{\vartheta}_2 Z_{it} \\
= \delta_0+ \sum{\sum{\delta_{ij} k_{t}^{i} i_{t}^{j}}} - \hat{\beta_l} l_{it} - \hat{\vartheta}_{1} X_{it} - \hat{\vartheta}_{2} Z_{it}
%\end{array}
\end{equation}
\vspace{1cm}

% -- case 2

\begin{equation}
\begin{array}{lll}
\hat{\varphi}_{it} (i_{it}, k_{it}) &=\hat{y}_{it} - \hat{\beta_l} l_{it} - \hat{\vartheta}_1 X_{it} - \hat{\vartheta}_2 Z_{it} \\
&= \delta_0+ \sum{\sum{\delta_{ij} k_{t}^{i} i_{t}^{j}}} - \hat{\beta_l} l_{it} - \hat{\vartheta}_{1} X_{it} - \hat{\vartheta}_{2} Z_{it}
\end{array}
\end{equation}
\vspace{1cm}

% --- case 3

\begin{equation}
\begin{aligned}
\hat{\varphi}_{it} (i_{it}, k_{it}) &= \hat{y}_{it} - \hat{\beta_l} l_{it} - \hat{\vartheta}_1 X_{it} - \hat{\vartheta}_2 Z_{it} \\
&= \delta_0+ \sum{\sum{\delta_{ij} k_{t}^{i} i_{t}^{j}}} - \hat{\beta_l} l_{it} - \hat{\vartheta}_{1} X_{it} - \hat{\vartheta}_{2} Z_{it}
\end{aligned}
\end{equation}

\end{document}