0
$\begingroup$

An elementary question about real inflection points of cubics:

Textbooks mention that non-singular cubics in $P^2(C)$ have 3 real and 6 complex inflections and show the Hesse normal form $ x^3 + y^3 + z^3 + 3m xyz=0 $, $m \in C $, $ m^3 \neq 1$, with the usual table of its 9 inflections $ (0, 1, -1), ..., (1, \exp(2 \pi i/3), 0)$ .

Does every non-singular cubic $ \ F(x,y,z) = \Sigma a_{i,j} x^i y^j z^{3-i-j} = 0 \ $ has 3 real inflections even if $a_{i,j}$ are complex valued? Or is it true only for real cubics with real $a_{i,j}$ ? Is there a reference which states this clearly?

$\endgroup$
1
  • $\begingroup$ Take your favourite cubic, e.g. $X^3+Y^3+Z^3=0$. It has 9 inflection points altogether. Choose a "random" linear transformation of $\mathbf P^2$ with complex coefficients. You will end up with a cubic (with complex coefficients) with no real inflection points. $\endgroup$ Commented Jul 8 at 13:48

0

You must log in to answer this question.

Browse other questions tagged .