By David Cox, Andrew R. Kustin, Claudia Polini, Bernd Ulrich
Think of a rational projective curve C of measure d over an algebraically closed box kk. There are n homogeneous types g1,...,gn of measure d in B=kk[x,y] which parameterise C in a birational, base element loose, demeanour. The authors learn the singularities of C via learning a Hilbert-Burch matrix f for the row vector [g1,...,gn]. within the ""General Lemma"" the authors use the generalised row beliefs of f to spot the singular issues on C, their multiplicities, the variety of branches at each one singular element, and the multiplicity of every department. permit p be a novel element at the parameterised planar curve C which corresponds to a generalised 0 of f. within the ""Triple Lemma"" the authors provide a matrix f' whose maximal minors parameterise the closure, in P2, of the blow-up at p of C in a neighbourhood of p. The authors practice the overall Lemma to f' in an effort to know about the singularities of C within the first neighbourhood of p. If C has even measure d=2c and the multiplicity of C at p is the same as c, then he applies the Triple Lemma back to benefit in regards to the singularities of C within the moment neighbourhood of p. reflect on rational aircraft curves C of even measure d=2c. The authors classify curves in line with the configuration of multiplicity c singularities on or infinitely close to C. There are 7 attainable configurations of such singularities. They classify the Hilbert-Burch matrix which corresponds to every configuration. The research of multiplicity c singularities on, or infinitely close to, a hard and fast rational aircraft curve C of measure 2c is such as the examine of the scheme of generalised zeros of the mounted balanced Hilbert-Burch matrix f for a parameterisation of C
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Extra resources for A study of singularities on rational curves via syzygies
9) holds if and only if rank A(λ) ≥ i for all non-zero λ ∈ As ⇐⇒ Ii (A(λ)) = 0 for all non-zero λ ∈ As ⇐⇒ ht Ii (A) ≥ s. The last equivalence is due to Hilbert’s Nullstellensatz which applies since k is algebraically closed. 10. 1 with n = 3, d = 2c, d1 = d2 = c, and k an algebraically closed ﬁeld. Let T and u be the row vectors T = [T1 , T2 , T3 ] and 30 3. THE BIPROJ LEMMA u = [u1 , u2 ] of indeterminates. 1). 12) T ] and the entries of A are linear forms so that the entries of C are linear forms in k [T u].
18. 10. The curve C has only singularities of multiplicity at most c − 1 if and only if every generalized column ideal of C has height three. Proof. 9. 14. 7, ht I3 (A) = 2 if and only if every generalized column ideal of C has height three. 19. 10 with c ≥ 2. The following statements hold: (1) ht I2 (C) ≥ 2, (2) ht I3 (A) ≥ 1, and (3) ht I1 (Cλ ) = 3 for general λ ∈ A2 . k [T T ]/I2 (C)) is either empty or is a Proof. 14 shows that Proj(k ﬁnite set; therefore, (1) and (2) hold. 7. 20. Let C be matrix of linear forms from a polynomial ring R in three variables over a ﬁeld k .
1 we describe how to read the inﬁnitely near singularities in the ﬁrst neighborhood of p from the Hilbert-Burch matrix ϕ for the parameterization of C. 1. 1. 1 with n = 3, and k an algebraically closed ﬁeld. Assume that p = [0 : 0 : 1] is a singular point on the curve C of multiplicity dj , for j equal to 1 or 2. Assume further that ⎤ ⎡ P1 Q1 ϕ = ⎣P2 Q2 ⎦ , 0 Q3 where the Pi and the Qi are homogeneous forms from B with deg Pi = d − dj and deg Qi = dj . Then, a point P = (p, [a : b]) ∈ P2 × P1 lies on the blowup of C at p if and only if gcd(Q3 , aP1 + bP2 ) is not a constant.