It comes from 有限群导引(上)-徐明曜.pdf, page 45. (i) If abab=1, then b=abab2 b2=aba a=a2bab a2=bab ab=a2bab2 ab2=a2ba a2b=bab2 a2b2=ba b2a=aba2 ba2=b2ab b2a2=ab ab2a=a2ba2 aba, a2ba, ab2a, aba2, ab2ab=a2baab=a2ba2b 1+4+8+16+32=61