Stirling's factorial approximation

extra/math/analysis/analysis.factor

@@ -108,3 +108,12 @@ PRIVATE>
         swap -1.0 * exp
         *
     ] if ;
+
+! James Stirling's approximation for N!:
+! http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Numerical/Stirling/
+
+: stirling-fact ( n -- fact )
+  [ pi 2 * * sqrt ]
+  [ dup e / swap ^ ]
+  [ 12 * recip 1 + ]
+  tri * * ;