deletions | additions       diff --git a/introduction.tex b/introduction.tex  index a6b8b0b..b2c4cf1 100644  --- a/introduction.tex  +++ b/introduction.tex 
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dP_\pm = (dP \pm \rho c dU)/2  \]  \[  dU\pm dU_\pm  = \pm (dP \pm \rho c dU)/2 \rho c \]  The net wave intensity does not depend upon the wave speed  \[