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diff --git a/math/bevington/factor.f b/math/bevington/factor.f
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+c function factor.f
+c
+c source
+c   Bevington, page 32.
+c
+c purpose
+c   calculates factorial function for integers
+c
+c usage
+c   result = factor (n)
+c
+c description of parameters
+c   n      - integer argument
+c
+c subroutines and function subprograms required
+c   none
+c
+	function factor (n)
+	double precision fi,sum
+11	factor=1.
+	if (n-1) 40,40,13
+13	if (n-10) 21,21,31
+c
+c n less than 11
+c
+21	do 23 i=2,n
+	fi=i
+23	factor=factor*fi
+	goto 40
+c
+c n greater than 10
+c
+31	sum=0.
+	do 34 i=11,n
+	fi=i
+34	sum=sum+dlog(fi)
+35	factor=3628800.*dexp(sum)
+40	return
+	end