Definition 14.31.1. A map X \to Y of simplicial sets is called a Kan fibration if for all k, n with 1 \leq n, 0 \leq k \leq n and any commutative solid diagram
\xymatrix{ \Lambda _ k[n] \ar[r] \ar[d] & X \ar[d] \\ \Delta [n] \ar[r] \ar@{-->}[ru] & Y }
a dotted arrow exists making the diagram commute. A Kan complex is a simplicial set X such that X \to * is a Kan fibration, where * is the constant simplicial set on a singleton.
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