Index: qd-gamma.lisp
===================================================================
--- qd-gamma.lisp	(revision 19dd5231c42c6eec40a7876bd9ffb2661fb05012)
+++ qd-gamma.lisp	(revision b1ae6953934670284060ffc2810e424a5e0aac71)
@@ -376,7 +376,8 @@
 	 (a (apply-contagion a prec))
 	 (z (apply-contagion z prec)))
-    (if (zerop a)
-	;; incomplete_gamma_tail(0, z) = exp_integral_e(1,z)
-	(exp-integral-e 1 z)
+    (if (and (realp a) (<= a 0))
+	;; incomplete_gamma_tail(v, z) = z^v*exp_integral_e(1-a,z)
+	(* (expt z a)
+	   (exp-integral-e (- 1 a) z))
 	(if (and (zerop (imagpart a))
 		 (zerop (imagpart z)))