http://www.physics.ucsb.edu/~jpierre/strings/dbranes.htm
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SUPERSTRINGS! D-branes
Strings can have various kinds of boundary conditions. For
example closed strings have periodic boundary conditions
(the string comes back onto itself). Open strings can have
two different kinds of boundary conditions called Neumann
and Dirichlet boundary conditions. With Neumann boundary
conditions the endpoint is free to move about but no
mo
