http://books.google.com/books?id=Yr2pJA950iAC&pg=PA140&lpg=PA140&dq=incidence+matrix+rank+of+a+graph&source=bl&ots=9expNEXB_S&sig=5qb8FNhNPFbAEi3taFMxN1WKHR4&hl=en&ei=cNzcTfD4N8vJsgbaleTpDg&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDAQ6AEwBQ#v=onepage&q=incidence%20matrix%20rank%20of%20a%20graph&f=false
