An important theorem in the study of subvarieties of projective space is the following theorem of Fulton and Hansen [FH]: If X n ⊂ PV is a variety, τ (X) its tangential variety and σ(X) its secant variety, then either dim τ (X) = 2n and dim σ(X) = 2n + 1 or τ (X) = σ(X) (see below for definitions). τ (X) is the union of tangent stars of X, but from the perspective of differential geometry , one...
