Invariant multidimensional matrices Jean Vall`es 4 0 0 2 “On se persuade mieux, pour l’ordinaire , par les raisons qu’on a soi-mˆeme trouv´ees que par celles n qui sont venues dans l’esprit des autres”(Pascal). a J Abstract : In [AO] the authors study Steiner bundles via their unstable hyperplanes and proved 4 that (see [AO], Tmm 5.9) : 1 A rank n Steiner bundle on Pn which is SL(2,C) invariant is a Schwarzenberger bundle. G] InthisnotewegiveaveryshortproofofthisresultbasedonClebsch-GordonproblemforSL(2,C)- modules. A . h 1 Introduction t a m Let A, B, and C three vector spaces over C and φ: A⊗B → C∗ a linear surjective map. [ We consider the sheaf S on P(A)×P(B) defined by, φ 1 v tφ 7 0 −−−−→ Sφ −−−−→ C ⊗OP(A)×P(B) −−−−→ OP(A)×P(B)(1,1) 4 1 with fibers S (a⊗b)= {c ∈ C |φ(a⊗b)(c) = 0}. 1 φ 0 Remark 1. If dimCC < dimCA+dimCB −1 then ker(φ) meets the set of decomposable 4 tensors, so there exist a∈ A , a 6= 0 and b ∈ B, b 6= 0 such that φ(a⊗b) = 0. 0 / Remark 2. When S is a vector bundle it gives two “associated” Steiner bundles S on h φ A t P(A) and S on P(B) after projections (see [DK], prop. 3.20). a B m We denote by (P(A) × P(B) × P(C))∨ the variety of hyperplanes tangent to the Segre v: P(A)×P(B)×P(C) and by ({a}×{b}×P(C))∨ the set of hyperplanes in P(A⊗B⊗C) i containing {a}×{b}×P(C). X r Thenextpropositionisareformulation ofmanyresultsfrom,[GKZ](seeforinstanceThm a 3.1’ page 458, prop 1.1 page 445), [AO] (see thm page 1) and [DO] (see cor.3.3). Proposition 1.1. Let A, B, and C three vector spaces over C with dimCA = n + 1, ∗ dimCB = m+1 and dimCC ≥ n+m+1 and φ : A⊗B → C a surjective linear map. Then the following propositions are equivalent : 1) S is a vector bundle over P(A)×P(B). φ 2) φ(a⊗b) 6= 0 for all a∈ A, a6= 0 and b ∈B, b 6= 0. 3) φ ∈/ ({a}×{b}×P(C))∨ for all a∈ A, a6= 0 and b ∈ B, b 6= 0. 4) φ ∈/ (P(A)×P(B)×P(C))∨ Remark 3. When dimCC = dimCA+dimCB −1 the variety (P(A)×P(B)×P(C))∨ is an hypersurface in P((A⊗B⊗C)∨). This hypersurface is defined by the vanishing of the 1 hyperdeterminant, say Det(Φ) where Φ is the generic tridimensional matrix (see [GKZ], chapter 1 and 14). Proof. It is clear that 1) 2) and 3) are equivalent. It remains to show that 3) and 4) are equivalent too. Since ∂(abc) = (∂(a))bc + a(∂(b))c + ab(∂(c)) an hyperplane H is tangent to the Segre in a point (a,b,c) if and only if it contains P(A)×{b}×{c} and {a}×P(B)×{c} and {a} × {b}× P(C). We prove here that the third condition implies the two others. Let H an hyperplane containing {a} × {b} × P(C), we show that there exists c ∈ C such that H contains P(A)×{b}×{c} and {a}×P(B)×{c}. Let φ the trilinear application correspondingtoH. Sinceφ(a⊗b)(C)= 0wehaveadimCC-dimensionalfamilyofbilinear forms vanishing on (a,b). Now finding a bilinear form of the above family (i.e. finding c ∈ C)whichverifyφ(A⊗b)(c) = 0andφ(a⊗B)(c) =0imposesat mostn+m conditions. Since dimCC ≥ n+m+1, this point c exists. (cid:3) 2 Invariant tridimensional matrix under SL(2,C)-action In the second part of this note we will consider the boundary case dimCC = dimCA+dimCB −1 Then, instead of writing φ induces a vector bundle or φ ∈/ (P(A)×P(B)×P(C))∨ we will write equivalently Det(φ) 6= 0. WedenotebySitheirreducibleSL(2,C)-representationsofdegreeiandby(xi−kyk)k=0,···,i a basis of S . i Theorem 2.1. Let A, B and C be three non trivial SL(2,C)-modules with dimension n+1, m+1 and n+m+1 and φ∈ P(A⊗B⊗C) an invariant hyperplane under SL(2,C). Then, Det(φ) 6= 0 ⇔ φ is the multiplication S ⊗S → S n m n+m Proof. When φ ∈ P(S ⊗S ⊗S ) is just the multiplication S ⊗S → S it is n m n+m n m n+m well known that it corresponds to Schwarzenberger bundles (see [DK], prop 6.3). Conversely, let A = ⊕i∈ISi ⊗ Ui, B = ⊕j∈JSj ⊗ Vj where Ui, Vj are trivial SL(2,C)- representations of dimension n and m . Let xi ∈ S , xj ∈ S be two highest weight i j i j vectors and u∈ U , v ∈ V . Since Det(φ) 6= 0, φ((xi⊗u)⊗(xj⊗v)) 6= 0 and by SL(2,C)- i j invariance φ((xi⊗u)⊗(xj⊗v))= xi+jφ(u⊗v)∈ S ⊗W . By hypothesis φ(u⊗v)6= 0 i+j i+j for all u∈ U and v ∈ V so, by the Remark 1, it implies that dimW ≥ n +m −1, and i j i+j i j Sni+mj−1 ⊂ C∗. i+j AssumenowthatB containsatleasttwodistinctirreduciblerepresentations. Leti andj 0 0 the greatest integers in I and J. We consider the submoduleB such that B ⊕Smj0 = B. 1 1 j0 ∗ Then the restricted map A⊗B → C is not surjective because the image is concentrated 1 in the submodule C∗ of C∗ defined by C∗⊕Sni0+mj0−1 =C∗. Now since 1 1 i0+j0 dimC(C1)< dimC(A)+dimC(B1)−1 there exist a∈ A, b ∈ B ⊂ B such that φ(a⊗b)= 0. A contradiction with the hypothesis 1 Det(φ) 6= 0. 2 So A = Sni, B = Smj and Sni+mj−1 ⊂ C∗. Since dimCC = dimCA+dimCB−1, we have i j i+j (i+1)ni+(j+1)mj−1 = dimCC ≥ (i+j+1)(ni+mj−1) which is possible if and only if n = m = 1 and C = S . (cid:3) i j i+j Corollary2.2. AranknSteinerbundleonPn whichisSL(2,C)invariantisaSchwarzen- berger bundle. Proof. Let S a rank n Steiner bundle on Pn, i.e S appears in an exact sequence ∗ 0 −−−−→ S −−−−→ C ⊗OP(A) −−−−→ B ⊗OP(A)(1) −−−−→ 0 where P(A) = Pn, P(B) = Pm and P(C)= Pn+m. If SL(2,C) acts on S the vector spaces A, B and C are SL(2,C)-modules since A is the basis, B∗ = H1S(−1) and C∗ = H0(S∗). If S is SL(2,C)-invariant the linear surjective map A⊗(H1S(−1))∗ → H0(S∗) is SL(2,C)-invariant too. (cid:3) Remark. The proofs of the theorem and the proposition, given in this paper, are still valid for more than three vector spaces when the format is the boundary format. I would like to thank L.Gruson, M.Meulien and N.Perrin for their help. References [AO] V. Ancona, G. Ottaviani. Unstable hyperplanes for Steiner bundles and multidi- mensional matrices, Advances in Geometry, 1,No.2, (2001), 165-192. [DO] C. Dionisi, G. OttavianiThe Binet-Cauchy theorem for the hyperdeterminant of boundary format multidimensional matrices math.AG/0104281, [DK] I. Dolgachev, M. M. KapranovArrangement of hyperplanes and vector bundles on Pn, Duke Math. J. (1993), no. 71, 633–664, [GKZ] I. M. Gelfand, M. M. Kapranov, and A. V. Zelevinsky, Discriminants, resul- tants, andmultidimensionaldeterminants.Mathematics: Theory&Applications. Birkh¨auser Boston, Inc., Boston, MA, (1994). ISBN: 0-8176-3660-9 [Sch] Schwarzenberger, R.L.E., Vector bundles on the projective plane, Proc. London Math. Soc. 11, (1961), 623-640. 3