Index: qd-gamma.lisp
===================================================================
--- qd-gamma.lisp	(revision 4b332ed2140e56c6fcaa689e6b8a48be36c988f6)
+++ qd-gamma.lisp	(revision 1d9ec007bb4172eaccb8ca1db543c1218192cdb5)
@@ -528,5 +528,10 @@
   ;;
   ;;
-  (cond ((< (abs z) 1)
+  (cond ((and (realp v) (minusp v))
+	 ;; E(-v, z) = z^(-v-1)*incomplete_gamma_tail(v+1,z)
+	 (let ((-v (- v)))
+	   (* (expt z (- v 1))
+	      (incomplete-gamma-tail (+ -v 1) z))))
+	((< (abs z) 1)
 	 ;; Use series for small z
 	 (s-exp-integral-e v z))