文档介绍：arXiv:hep-th/0007175 v1 21 Jul 2000
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1. Introduction And Definition Of K(X)
A D-brane wrapped on a submanifold S of spacetime may carry a nonzero Ramond-
Ramond (RR) charge. RR fields are p-forms, and superficially it seems that the conserved
charge should be measured by the cohomology class of the RR form (or of the cycle S itself).
However, D-branes carry gauge fields; and gauge fields are not natural in (co)homology
theory. They are natural in “K-theory.” K-theory has been used to answer some questions
about RR charges and fields; the aim of the present article1 is to give an overview of this,
along with some speculations (for which I have only very modest evidence) about how one
might want to rethink open string field theory in the large N limit. These subjects fill
sections 1-3. Some mathematical details