Index: qd-gamma.lisp
===================================================================
--- qd-gamma.lisp	(revision e10e402d446339130042607357b0129b35a1faa7)
+++ qd-gamma.lisp	(revision fe8cffb5ee8e7161addf7586ecaee00682c6bf1b)
@@ -498,16 +498,17 @@
 	     (= v (ftruncate v)))
 	;; v is an integer
-	(let ((n (truncate v)))
-	  (- (* (/ (expt -z (- v 1))
+	(let* ((n (truncate v))
+	       (n-1 (1- n)))
+	  (- (* (/ (expt -z n-1)
 		   (gamma v))
 		(- (psi v) (log z)))
 	     (loop for k from 0
-		       for term = 1 then (* term (/ -z k))
-		   for sum = (/ (- 1 v)) then (+ sum (let ((denom (+ k 1 (- n))))
+		   for term = 1 then (* term (/ -z k))
+		   for sum = (/ (- 1 v)) then (+ sum (let ((denom (- k n-1)))
 						       (if (zerop denom)
 							   0
-							   (/ term (+ k 1 -v)))))
-		       when (< (abs term) (* (abs sum) eps))
-			 return sum)))
+							   (/ term denom))))
+		   when (< (abs term) (* (abs sum) eps))
+		     return sum)))
 	(loop for k from 0
 	      for term = 1 then (* term (/ -z k))