The virtual class in orbifold and stringy cohomology and
K-theory -- Tyler Jarvis

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We derive a simple formula for computing the obstruction bundle
and the virtual class in orbifold and stringy cohomology and
K-theory.  This allows us to give a simple, alternative,
definition of orbifold (and stringy) cohomology and K-theory,
allows us to prove that the orbifold (and stringy) Chern
character is a ring isomorphism, and greatly simplifies
computation of the orbifold cohomology.