Manuscript received November 8;
1993;
revised August 20;
1995. Recommended by Associate Editor;
S. P. Meyn. This work was supported in part by the NSF under Grants ECS-9102714;
ECS-9113029;
and DMS-9305936. T. E. Duncan and B. Pasik-Duncan are with the Department of Mathematics;
University of Kansas;
'Lawrence;
KS 66045 USA. P. Mandl is with the Department of Probability and Mathematical Statistics;
Charles University;
Prague;
Czech Republic. Publisher Item Identifier S 0018-9286(96)02827-9;