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+c function gamma.f
+c
+c source
+c   Bevington, page 126.
+c
+c purpose
+c   calculate the gamma function for integers and half-integers
+c
+c usage
+c   result = gamma (x)
+c
+c description of parameters
+c   x      - integer or half-integer
+c
+c subroutines or function subprograms required
+c   factor (n)
+c      calculates n factorial for integers
+c
+	function gamma (x)
+	double precision prod,sum,fi
+c
+c integerize argument
+c
+11	n=x-0.25
+	xn=n
+13	if (x-xn-0.75) 31,31,21
+c
+c argument is integer
+c
+21	gamma=factor(n)
+	goto 60
+c
+c argument is half-integer
+c
+31	prod=1.77245385
+	if (n) 44,44,33
+33	if (n-10) 41,41,51
+41	do 43 i=1,n
+	fi=i
+43	prod=prod*(fi-0.5)
+44	gamma=prod
+	goto 60
+51	sum=0.
+	do 54 i=11,n
+	fi=i
+54	sum=sum+dlog(fi-0.5)
+55	gamma=prod*639383.8623*dexp(sum)
+60	return
+	end