  We investigate the dynamics of how graduate students coordinate their
mathematics and physics knowledge within the context of solving a homework
problem for a plasma physics survey course. Students were asked to obtain the
complex dielectric function for a plasma with a specified distribution function
and find the roots of that expression. While all the 16 participating students
obtained the dielectric function correctly in one of two equivalent
expressions, roughly half of them (7 of 16) failed to compute the roots
correctly. All seven took the same initial step that led them to the incorrect
answer. We note a perfect correlation between the specific expression of
dielectric function obtained and the student's success in solving for the
roots. We analyze student responses in terms of a resources framework and
suggest routes for future research.
