Arnaud Durand Miki Hermann Phokion G. Kolaitis Subtractive Reductions and Complete Problems for Counting Complexity Classes. 323-332 http://dx.doi.org/10.1007/3-540-44612-5_28 2000 conf/mfcs/2000 MFCS db/conf/mfcs/mfcs2000.html#DurandHK00