The Categorified Heisenberg Algebra I: A Combinatorial Representation
  We give an introductory account of Khovanov's categorification of the Heisenberg algebra, and construct a combinatorial model for it in a 2-category of spans of groupoids. We also treat a categorification of $U(sl_n)$ in a similar way. These give rise to standard representations on vector spaces through a linearization process. 
Jeffrey C. Morton, Jamie Vicary
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