Degenerate cubic containing a line
= Degenerate cubic containing a line
{title2=$L^3=0$}
A projective line $L=0$ is contained in the zero locus of the nonzero cubic $L^3$. The union of three lines is given by their product, and a conic together with a line is likewise a <plane cubic>. Reducibility and repeated factors are permitted in polynomial coverings used in <incidence geometry>; a covering by cubics need not consist of smooth irreducible curves.