sentences:  The next center of blowing up "C"1 is given by "x"="y"=0.However, the strict transform of "X" is "X"1, which is generated by "x"-"y"2 and "y"2+"z"2-"w"3.This means that the intersection of "C"1 and "X"1 is given by "x"="y"=0 and "z"2-"w"3=0, which is not regular.