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A circle bundle 
is a fiber bundle whose fibers 
are circles. It may also have the structure of a principal bundle if there is an action of 
that preserves the fibers, and is locally trivial. That is, if every point has a trivialization 
such that the action of 
on 
is the usual one.
SEE ALSO:Bundle, Group Action, Principal Bundle
This entry contributed by Todd Rowland
