% Bernd Berg sep 21 2017.
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\centerline{\bf Special and General Relativity (PHZ 4601/5606) Fall 2017 
Solutions} \medskip

\centerline{\bf Set 6} 

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% \end{figure} \centerline{Potential as function of $z$.}

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\item{\bf 15. Contractions.}
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${\rm (1),\ (2)\ and\ (3)}\!:\ \ 25-1-4-9=11$;~~
${\rm (4),\ (5)\ and\ (6)}\!:\ \ 25-9-16=0$; \smallskip

${\rm (7)}\!:\ \ 25-4-9-16=-4$.
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