Smooth points on irreducible hypersurfaces over an algebraically closed field. The characteristic bound supplies a nonzero partial derivative; the Nullstellensatz supplies a point where it does not vanish.
theorem
CCDN.exists_hypersurface_point_eval_ne_zero
{K : Type u_1}
{σ : Type u_2}
[Field K]
[IsAlgClosed K]
[Finite σ]
(f g : MvPolynomial σ K)
(hf : Irreducible f)
(hg : ¬f ∣ g)
:
∃ (z : σ → K), (MvPolynomial.eval z) f = 0 ∧ (MvPolynomial.eval z) g ≠ 0
A polynomial outside a prime hypersurface ideal is nonzero at some point of that hypersurface over an algebraically closed field.
theorem
CCDN.exists_smooth_hypersurface_point_of_degree_lt_char
{K : Type u_1}
{σ : Type u_2}
[Field K]
[IsAlgClosed K]
[Finite σ]
(f : MvPolynomial σ K)
(hf : Irreducible f)
(hpos : 0 < f.totalDegree)
(hchar : CharZeroOrGT K f.totalDegree)
:
∃ (z : σ → K) (j : σ), (MvPolynomial.eval z) f = 0 ∧ (MvPolynomial.eval z) ((MvPolynomial.pderiv j) f) ≠ 0
An irreducible hypersurface of positive degree smaller than the characteristic (or in characteristic zero) has a smooth rational point over the algebraically closed base.