Alfredo De Santis Giovanni Di Crescenzo Oded Goldreich Giuseppe Persiano The Graph Clustering Problem has a Perfect Zero-Knowledge Proof Electronic Colloquium on Computational Complexity (ECCC) 1998 6 5 db/journals/eccc/eccc5.html#ECCC-TR98-006 http://eccc.hpi-web.de/eccc-reports/1998/TR98-006/index.html