（ab2+a2b）^1/3
=(ab2)1/3+3(ab2)2/3+3(a2b)2/3+(a2b)1/3
=a1/3b2/3+3a2/3b4/3+3a4/3b2/3+a2/3b1/3
=a1/3b1/3(b1/3+3a1/3b+3ab1/3+a1/3)