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One of the most revolutionary ideas Einstein developed in his 
miraculous year, 1905, was the relationship between mass and 
energy, as embodied in the equation
\begin{equation}
	E = mc^2
	\label{eq:Einstein} % this label allows you to refer to the 
						% equation by number elsewhere in the text
\end{equation}
This equation implies that a change in a body's energy entails a
change in its inertial mass. Without doubt,
Eq.~(\ref{eq:Einstein}) is one of the most significant ideas of
physics in the 20th century.

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