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Bilinear form

In mathematics, particularly in linear algebra, a bilinear form on a vector space V over a field F is a function B: V \times V \to F that is linear in each of its two arguments separately, meaning B(av + bw, u) = aB(v, u) + bB(w, u) and B(u, av + bw) = aB(u, v) + bB(u, w) for all scalars a, b \in F and vectors u, v, w \in V.^[1]^[2] Bilinear forms generalize the concept of inner products and play a central role in encoding geometric and algebraic structures on vector spaces. They can be represented by matrices with respect to a basis of V: if \{e_1, \dots, e_n\} is a basis, then B(v, w) = ^T M , where M = (B(e_i, e_j)) is an n \times n matrix over F, and , are the coordinate vectors of v and w.^[1]^[2] Key properties include symmetry, where B(v, w) = B(w, v) for all v, w \in V (corresponding to a symmetric matrix M^T = M), and skew-symmetry or alternation, where B(v, w) = -B(w, v) or B(v, v) = 0 for all v, w \in V (corresponding to a skew-symmetric matrix M^T = -M with zero diagonal).^[1] A bilinear form is non-degenerate if its matrix is invertible, ensuring no nonzero vector annihilates the form, which is crucial for preserving dimensions in associated dual spaces.^[1]^[3] Symmetric bilinear forms are intimately linked to quadratic forms: given a quadratic form Q: V \to F satisfying Q(cv) = c^2 Q(v) and the parallelogram law, there is a unique associated symmetric bilinear form via the polarization identity B(v, w) = \frac{1}{4} [Q(v + w) - Q(v - w)], and conversely Q(v) = B(v, v) when the characteristic of F is not 2.^[1]^[2] Positive definite symmetric bilinear forms, such as the standard dot product on \mathbb{R}^n, define norms \|v\| = \sqrt{B(v, v)} and angles, enabling the spectral theorem for symmetric matrices, which guarantees orthonormal bases of eigenvectors.^[2] Beyond linear algebra, bilinear forms appear in diverse applications, including differential geometry (e.g., metrics on tangent spaces), representation theory (e.g., invariant forms on Lie algebras like the Killing form), and number theory (e.g., in estimates involving Kloosterman sums and modular forms).^[1]^[4] Their study extends to classification over specific fields, such as real or finite fields, influencing topics from quadratic space decompositions to arithmetic progressions of primes.^[1]^[5]

Basic Concepts

Definition

A bilinear form on a vector space V over a field F is a mapping B: V \times V \to F that is linear in each argument when the other is held fixed.^[1] This bilinearity is expressed by the following axioms: for all scalars a, b \in F and all vectors u, v, w \in V,

\begin{align} B(au + bv, w) &= a B(u, w) + b B(v, w), \\ B(u, av + bw) &= a B(u, v) + b B(u, w). \end{align}

These conditions ensure additivity and homogeneity in each variable separately.^[1] The set of all such bilinear forms on V is denoted \mathrm{Bil}(V), which itself forms a vector space under pointwise addition and scalar multiplication.^[6] More generally, one may consider bilinear maps B: V \times W \to F between two vector spaces, but the standard case focuses on V = W.^[1] Typically, V is taken over the real numbers \mathbb{R} or complex numbers \mathbb{C}, with no additional structure such as a norm imposed at the outset.^[7] The concept assumes only the algebraic framework of a vector space, allowing bilinear forms to generalize structures like the dot product without requiring metric properties. The notion of bilinear forms traces back to the late 18th century in Joseph-Louis Lagrange's implicit use of matrices to study quadratic forms, and the term was introduced in the context of multilinear algebra during the early 20th century as vector spaces were formalized.^[8]

Examples

A fundamental example of a symmetric bilinear form is the standard dot product on the Euclidean space \mathbb{R}^n, defined by B(\mathbf{u}, \mathbf{v}) = \sum_{i=1}^n u_i v_i for \mathbf{u} = (u_1, \dots, u_n) and \mathbf{v} = (v_1, \dots, v_n).^[9] This form arises naturally in the study of inner product spaces and captures the notion of orthogonality when B(\mathbf{u}, \mathbf{v}) = 0.^[1] Another illustrative example is the determinant form on the vector space \mathbb{R}^2, given by B(\mathbf{x}, \mathbf{y}) = \det\begin{pmatrix} x_1 & y_1 \\ x_2 & y_2 \end{pmatrix} = x_1 y_2 - x_2 y_1, where \mathbf{x} = (x_1, x_2) and \mathbf{y} = (y_1, y_2).^[10] This alternating bilinear form, which changes sign upon swapping arguments, is used to define oriented areas in the plane and exemplifies non-symmetric behavior. In the context of special relativity, the Minkowski metric provides an example of an indefinite bilinear form on \mathbb{R}^{1,3}, defined by B(\mathbf{u}, \mathbf{v}) = u_1 v_1 + u_2 v_2 + u_3 v_3 - u_4 v_4, where the indices correspond to spatial and temporal components.^[11] This form, with signature (3,1), measures proper time and length in four-dimensional spacetime while allowing both positive and negative values.^[12] The zero form, B(\mathbf{u}, \mathbf{v}) = 0 for all \mathbf{u}, \mathbf{v} in a vector space, represents the trivial bilinear form, which satisfies bilinearity but lacks non-trivial structure.^[1] Scalar multiples of non-trivial forms, such as B(\mathbf{u}, \mathbf{v}) = c \sum_{i=1}^n u_i v_i for a constant c \in \mathbb{R}, preserve bilinearity and scale the original form's values without altering its qualitative properties.^[1] Over finite fields, bilinear forms exhibit field-dependent behavior; for instance, on the vector space \mathbb{F}_{q^n} over \mathbb{F}_q, the trace form B(a, b) = \mathrm{Tr}_{\mathbb{F}_{q^n}/\mathbb{F}_q}(a b) uses the field trace to pair extension elements bilinearly.^[1] This form is symmetric and plays a key role in coding theory and the classification of forms in characteristic p.

Representation

Coordinate representation

In finite-dimensional vector spaces over a field F, bilinear forms admit a concrete coordinate representation with respect to a chosen basis. Let V be an n-dimensional vector space with basis \{ e_1, \dots, e_n \}. For a bilinear form B: V \times V \to F, the associated matrix A = (a_{ij})_{1 \leq i,j \leq n} is defined by a_{ij} = B(e_i, e_j). This matrix fully determines B, as the bilinearity ensures that for any u = \sum_{i=1}^n u_i e_i and v = \sum_{j=1}^n v_j e_j, the value is B(u, v) = \sum_{i=1}^n \sum_{j=1}^n u_i a_{ij} v_j = \mathbf{u}^T A \mathbf{v}, where \mathbf{u} = (u_1, \dots, u_n)^T and \mathbf{v} = (v_1, \dots, v_n)^T are the coordinate column vectors.^[1]^[13] To compute the matrix A, evaluate B directly on the basis vectors: the (i,j)-entry is simply B(e_i, e_j). This yields a straightforward procedure, as only n^2 evaluations are needed, after which the form on arbitrary vectors follows from expanding in coordinates and applying bilinearity. For instance, in the standard basis of F^n, the matrix entries correspond to the coefficients in the expression B(\mathbf{u}, \mathbf{v}) = \sum_{i,j} a_{ij} u_i v_j.^[1] The representation changes under a basis transformation. Suppose \{ f_1, \dots, f_n \} is a new basis, and let P be the invertible change-of-basis matrix whose k-th column consists of the coordinates of f_k with respect to the old basis \{ e_i \}. Then, the coordinate vectors transform as \mathbf{u} = P \mathbf{u}' and \mathbf{v} = P \mathbf{v}', where primes denote new coordinates. Substituting gives B(u, v) = (P \mathbf{u}')^T A (P \mathbf{v}') = {\mathbf{u}'}^T (P^T A P) \mathbf{v}', so the new matrix is A' = P^T A P. This congruence transformation preserves the bilinear form up to the choice of basis.^[1]^[13] Such matrix representations rely on the finite dimensionality of V, which ensures a finite basis and thus a finite matrix. In infinite-dimensional vector spaces, a similar coordinate approach is possible using a Hamel basis, leading to an infinite matrix with entries B(e_i, e_j), but these are typically unwieldy due to the lack of finite support in expansions, and practical treatments often defer to operator-theoretic or kernel representations instead.^[1]^[14]

Matrix properties

The rank of a bilinear form B on a finite-dimensional vector space V over a field F is the rank of its matrix representation A with respect to any basis of V.^[15] This quantity, denoted \operatorname{rank}(B), is independent of the choice of basis and equals the dimension of the image of the associated linear map \phi_B: V \to V^*, where V^* is the dual space of V and \phi_B(v)(w) = B(v, w) for all v, w \in V.^[15] In the symmetric case with \operatorname{char} F \neq 2, the rank also corresponds to the number of nonzero entries in a diagonal matrix obtained by congruence transformation.^[15] Two matrices A and B' represent the same bilinear form on V with respect to different bases if and only if B' = P^T A P for some invertible matrix P; matrices related in this manner are said to be congruent.^[16] Congruence preserves essential properties of the bilinear form, such as its rank, and provides a framework for classifying forms up to change of basis. For symmetric bilinear forms over the real numbers, Sylvester's law of inertia further refines this classification: the inertia of the form, expressed as the triple (p, q, r) where p is the number of positive eigenvalues of A, q the number of negative eigenvalues, and r the multiplicity of the zero eigenvalue (with p + q + r = \dim V), remains invariant under congruence.^[17] The signature of the form is the pair (p, q), which captures the "indefiniteness" structure.^[1] Symmetric bilinear forms admit diagonalization by congruence over fields of characteristic not equal to 2, meaning there exists a basis of V in which the matrix representation is diagonal.^[18] This holds in particular over algebraically closed fields, such as the complex numbers, where the diagonal entries can often be normalized to 1 for nondegenerate forms.^[18] For nondegenerate bilinear forms, the associated matrix A has full rank, implying \det A \neq 0.^[1] In the real symmetric nondegenerate case, the sign of the determinant is (-1)^q, where q is the number of negative eigenvalues (positive if q even, negative if q odd), but the signature provides a more complete invariant than the determinant alone.^[1]

Core Properties

Non-degeneracy

A bilinear form B: V \times V \to F on a vector space V over a field F is defined to be non-degenerate, or regular, if the associated linear map \phi_B: V \to V^*, given by \phi_B(u)(v) = B(u, v) for all u, v \in V, is injective; this is termed left non-degeneracy.^[1] Equivalently, the left kernel of B, denoted \{u \in V \mid B(u, v) = 0 \ \forall v \in V\}, is the zero subspace \{0\}.^[1] Similarly, right non-degeneracy holds if the map \psi_B: V \to V^*, defined by \psi_B(v)(u) = B(u, v), is injective, which is equivalent to the right kernel \{v \in V \mid B(u, v) = 0 \ \forall u \in V\} being \{0\}.^[19] When both left and right non-degeneracy conditions are satisfied, B is said to be non-degenerate in the proper sense.^[19] The radical of B, denoted \mathrm{rad}(B), is the intersection of the left kernel and the right kernel of B.^[20] Thus, B is non-degenerate if and only if \mathrm{rad}(B) = \{0\}.^[20] This characterization unifies the left and right conditions, emphasizing the form's ability to pair vectors non-trivially in both directions. In the finite-dimensional case, where \dim V = n < \infty, the spaces V and V^* have equal dimension n, so left non-degeneracy (injectivity of \phi_B) implies that \phi_B is an isomorphism, and similarly for right non-degeneracy.^[1] Consequently, B is non-degenerate if and only if \phi_B (or equivalently \psi_B) is an isomorphism from V to V^*. With respect to a basis of V, this holds if and only if the matrix A representing B has full rank n, i.e., \mathrm{rank}(A) = n.^[1] Representative examples illustrate these concepts. The standard dot product on \mathbb{R}^n, defined by B(\mathbf{x}, \mathbf{y}) = \mathbf{x} \cdot \mathbf{y} = \sum_{i=1}^n x_i y_i, is non-degenerate, as its left kernel (and right kernel, since it is symmetric) is \{[0](/page/0)\}—if \mathbf{x} \cdot \mathbf{y} = 0 for all \mathbf{y} \in \mathbb{R}^n, then \mathbf{x} = \mathbf{0}.^[1] In contrast, the zero form B(\mathbf{u}, \mathbf{v}) = 0 for all \mathbf{u}, \mathbf{v} \in V is degenerate, with both kernels equal to the entire space V and radical V \neq \{0\} (unless \dim V = 0).^[1]

Symmetry types

Bilinear forms are classified into symmetry types based on their behavior under interchange of arguments. A bilinear form B: V \times V \to F on a vector space V over a field F is symmetric if B(u, v) = B(v, u) for all u, v \in V.^[21] In terms of matrix representation with respect to a basis, a symmetric bilinear form corresponds to a symmetric matrix A, satisfying A^T = A.^[22] A bilinear form is skew-symmetric (or antisymmetric) if B(u, v) = -B(v, u) for all u, v \in V.^[23] The associated matrix representation is then skew-symmetric, with A^T = -A.^[23] Skew-symmetric forms satisfy B(u, u) = 0 for all u \in V, but the converse holds only over fields of characteristic not equal to 2.^[1] An alternating bilinear form is defined by the condition B(u, u) = 0 for all u \in V.^[1] Over fields of characteristic not equal to 2, every alternating form is skew-symmetric, but the converse is false in characteristic 2, where skew-symmetric forms need not vanish on the diagonal.^[1] For non-degenerate alternating forms over the real numbers, non-trivial examples exist only when the dimension of V is even.^[1] For a non-degenerate skew-symmetric bilinear form on a finite-dimensional even-dimensional vector space, the Pfaffian provides a polynomial invariant that serves as a square root of the determinant of the associated skew-symmetric matrix.^[24] Specifically, if A is the skew-symmetric matrix representing the form, then \operatorname{Pf}(A)^2 = \det(A).^[24]

Reflexivity and orthogonality

A bilinear form B on a vector space V over a field K is called reflexive if B(u, v) = 0 for all v \in V implies u = 0, and symmetrically, if B(u, v) = 0 for all u \in V implies v = 0.^[1] This condition ensures that the form captures the full structure of the space without collapsing any nonzero vectors to zero under orthogonalization. In finite-dimensional spaces, reflexivity is equivalent to the standard notion of non-degeneracy for bilinear forms.^[1] Bilateral non-degeneracy, or reflexivity, requires that both the left kernel \{ u \in V \mid B(u, v) = 0 \ \forall v \in V \} and the right kernel \{ v \in V \mid B(u, v) = 0 \ \forall u \in V \} are trivial, i.e., equal to \{0\}.^[1] This bilateral condition distinguishes reflexive forms from those that may be degenerate on one side only, ensuring a balanced duality between the two arguments. In coordinate terms, if V is finite-dimensional with basis \{e_i\}, the matrix representation A of B satisfies A_{ij} = B(e_i, e_j), and reflexivity holds if and only if \det A \neq 0.^[1] Given a reflexive bilinear form B, two vectors u, v \in V are orthogonal, denoted u \perp v, if B(u, v) = 0.^[1] For a subspace S \subseteq V, the orthogonal complement is the set S^\perp = \{ w \in V \mid B(u, w) = 0 \ \forall u \in S \}.^[1] Reflexivity implies that \dim S + \dim S^\perp = \dim V in finite dimensions, and S \cap S^\perp = \{0\} if S is non-degenerate as a restriction of B.^[1] For a non-degenerate symmetric bilinear form over the real numbers, the spectral theorem guarantees an orthogonal basis of eigenvectors, allowing V to decompose as an orthogonal direct sum of eigenspaces: V = \bigoplus_{\lambda \in \sigma(B)} E_\lambda, where E_\lambda = \{ u \in V \mid B(u, v) = \lambda \langle u, v \rangle for some inner product, but adapted to the form's geometry, with E_\lambda \perp E_{\mu} for \lambda \neq \mu.^[25] This decomposition highlights the form's role in classifying quadratic structures via eigenvalues. In the context of duality induced by a reflexive bilinear form B: V \times V \to K, which identifies V with its dual V^* via v \mapsto B(v, \cdot), the orthogonal complement S^\perp of a subspace S \subseteq V is precisely the annihilator of S under this pairing.^[1] Thus, reflexivity ensures the duality map is an isomorphism, preserving annihilator properties such as (S^\perp)^\perp = S for finite-dimensional V.^[1]

Advanced Properties

Boundedness

In normed vector spaces, a bilinear form B: V \times V \to \mathbb{F} (where \mathbb{F} is \mathbb{R} or \mathbb{C}) on a space (V, \|\cdot\|) is said to be bounded if there exists a constant C > 0 such that |B(u, v)| \leq C \|u\| \|v\| for all u, v \in V.^[26] This condition ensures that the form does not grow faster than the product of the norms of its arguments.^[27] The operator norm of a bounded bilinear form B is defined as \|B\| = \sup \{ |B(u, v)| : \|u\| \leq 1, \|v\| \leq 1 \}, which is finite and equivalent to the infimum of all such constants C.^[26] This norm measures the maximum "amplification" of the form on the unit ball. Boundedness is precisely equivalent to continuity of B as a map from V \times V (equipped with the product norm) to \mathbb{F}.^[26]^[27] In finite-dimensional normed spaces, every bilinear form is bounded. This follows from the fact that every linear operator between finite-dimensional normed spaces is continuous (and hence bounded), and a bilinear form induces linear operators by fixing one argument—for instance, the map v \mapsto B(u, v) is linear in v for fixed u, with boundedness uniform across unit vectors due to norm equivalence in finite dimensions.^[28] More generally, a bounded bilinear form B induces bounded linear operators, such as the left multiplication map V \to L(V, \mathbb{F}) given by u \mapsto (v \mapsto B(u, v)), which has operator norm equal to \|B\|.^[26] A prominent example of a bounded bilinear form is an inner product on a Hilbert space, which satisfies |\langle u, v \rangle| \leq \|u\| \|v\| by the Cauchy-Schwarz inequality, yielding operator norm at most 1.^[27]

Ellipticity

In the context of Hilbert spaces, a symmetric bilinear form B: H \times H \to \mathbb{R} is said to be elliptic, or coercive, if there exists a constant \alpha > 0 such that

B(u, u) \geq \alpha \|u\|^2

for all u \in H.^[29] This condition ensures that the form provides a lower bound relative to the norm induced by the inner product on H, often used to establish stability in variational problems.^[30] Over the real numbers, for a finite-dimensional inner product space with matrix representation A of B, ellipticity is equivalent to A being positive definite, meaning all eigenvalues of A are positive.^[1] This spectral characterization follows from the spectral theorem for symmetric matrices, where the quadratic form B(u, u) = u^T A u is bounded below by the smallest eigenvalue times \|u\|^2.^[1] In more general settings, such as Sobolev spaces H^1_0(\Omega) for bounded domains \Omega \subset \mathbb{R}^n, full coercivity may not hold due to lower-order terms, leading to Gårding's inequality.^[31] For a bilinear form B[u, v] = \int_\Omega \sum_{i,j=1}^n a_{ij} \partial_i u \partial_j v + \sum_{i=1}^n b_i (\partial_i u) v + c u v \, dx associated with a second-order elliptic operator in divergence form, there exist constants \alpha, \beta > 0 and \gamma \geq 0 such that

|B[u, v]| \leq \alpha \|u\|_{H^1_0(\Omega)} \|v\|_{H^1_0(\Omega)}

and

B[u, u] \geq \beta \|u\|_{H^1_0(\Omega)}^2 - \gamma \|u\|_{L^2(\Omega)}^2

for all u, v \in H^1_0(\Omega), assuming uniform ellipticity of the principal coefficients a_{ij}.^[31] This weakened coercivity, combined with compactness from the Rellich-Kondrachov theorem, enables solvability via the Fredholm alternative or modified Lax-Milgram arguments.^[32] Elliptic bilinear forms play a central role in the analysis of partial differential equations (PDEs), particularly in ensuring well-posedness of weak solutions.^[29] By the Lax-Milgram theorem, if B is continuous (bounded) and elliptic on a Hilbert space H, then for any bounded linear functional f \in H^*, the variational problem B(u, v) = \langle f, v \rangle for all v \in H admits a unique solution u \in H.^[30] For second-order uniformly elliptic PDEs like -\sum_{i,j} \partial_i (a_{ij} \partial_j u) + \sum_i b_i \partial_i u + c u = f with suitable boundary conditions, the associated bilinear form satisfies these hypotheses in H^1_0(\Omega), yielding unique weak solutions.^[30] In continuum mechanics, particularly for hyperelastic materials, ellipticity conditions on the tangent bilinear form distinguish between strong and weak variants.^[33] Strong ellipticity requires the bilinear form to be positive definite on the entire space of admissible symmetric tensor fields, ensuring the acoustic tensor has positive eigenvalues for all wave directions and guaranteeing material stability under arbitrary infinitesimal deformations.^[33] Weak ellipticity, in contrast, demands positivity only along rank-one directions (corresponding to plane waves), which is a necessary but insufficient condition for full stability and aligns with the Legendre-Hadamard condition but permits loss of ellipticity in higher-rank perturbations.^[33]

Associated Structures

Quadratic forms

A quadratic form associated with a bilinear form B: V \times V \to F on a vector space V over a field F is defined by Q(u) = B(u, u) for all u \in V.^[2] This association holds for any bilinear form, but when B is symmetric (i.e., B(u, v) = B(v, u) for all u, v \in V), the quadratic form Q uniquely determines B via the polarization identity, provided the characteristic of F is not 2:

B(u, v) = \frac{1}{4} \left[ Q(u + v) - Q(u - v) \right].

This identity follows from expanding Q(u + v) and Q(u - v) using bilinearity and collecting terms.^[11]^[1] More generally, over fields of characteristic not 2, the polarization identity can be expressed as

B(u, v) = \frac{1}{4} \sum_{\varepsilon = \pm 1} \varepsilon Q(u + \varepsilon v),

where the sum accounts for the contributions from the expansions.^[34] Over the complex numbers, where bilinear forms are often replaced by sesquilinear forms (linear in the first argument and conjugate-linear in the second), the polarization identity is adjusted to recover the sesquilinear form from the associated quadratic form \Phi(u) = \langle u, u \rangle, for example:

$4 \langle u, v \rangle = \Phi(u + v) - \Phi(u - v) + i \Phi(u + i v) - i \Phi(u - i v).

This ensures compatibility with the Hermitian structure.^[35]^[36] The quadratic form Q satisfies quadratic homogeneity: Q(a u) = a^2 Q(u) for all scalars a \in F and u \in V, which follows directly from the bilinearity of B.^[2] Over the real numbers, a quadratic form Q is positive semi-definite if Q(u) \geq 0 for all u \in V.^[37] In coordinates with respect to a basis, if B has matrix A = (B(e_i, e_j)), then Q(u) = ^T A for the column vector $$ of coordinates of u. For symmetric B, A is symmetric.^[11] For real symmetric bilinear forms, the associated quadratic form Q admits a classification via its signature, defined as the pair (p, q) where p is the number of positive eigenvalues, q the number of negative eigenvalues (with the rest zero), of the diagonalized form under Sylvester's law of inertia. This signature is invariant under change of basis and determines the inertia of Q.^[16]^[38]

Tensor product relations

Bilinear forms on a vector space V over a field F can be understood through their connection to tensor products in multilinear algebra. Specifically, for finite-dimensional V, the space of bilinear forms \mathrm{Bil}(V, V; F) is naturally isomorphic to the tensor product of the dual space with itself, V^* \otimes V^*. Under this isomorphism, a bilinear form B corresponds to an element \sum_i \phi_i \otimes \psi_i \in V^* \otimes V^*, where B(v, w) = \sum_i \phi_i(v) \psi_i(w) for all v, w \in V. In a basis \{e_j\} for V with dual basis \{e_i^*\} for V^*, the bilinear form expands as B = \sum_{i,j} B(e_i, e_j) e_i^* \otimes e_j^*.^[39] This tensor product perspective aligns with the universal property of the tensor product. A bilinear form B: V \times V \to F is equivalently a linear map from the tensor product V \otimes V to F, and thus an element of the dual space (V \otimes V)^*. The universal property ensures that \mathrm{Hom}(V \otimes V, F) \cong \mathrm{Bil}(V, V; F), providing a basis-free characterization that extends the algebraic structure of bilinear forms.^[40] For bilinear forms B: V \times W \to F on distinct finite-dimensional spaces V and W, the tensor isomorphism becomes \mathrm{Bil}(V, W; F) \cong V^* \otimes W^*, with B(v, w) = \sum_i \phi_i(v) \psi_i(w). This induces adjoint maps: B defines a linear map V \to W^* by v \mapsto (w \mapsto B(v, w)), and dually W \to V^* by w \mapsto (v \mapsto B(v, w)). These adjoints embed bilinear forms into the space of linear maps involving duals, facilitating connections in representation theory and duality.^[41] Bilinear forms represent the case k=2 of multilinear forms, which are linear maps on k-fold products V^k \to F, or elements of (V^{\otimes k})^*. The universal property generalizes: the tensor product V^{\otimes k} linearizes k-linear maps, so \mathrm{Bil}(V, V; F) is the special case where the multilinear form is separately linear in two arguments. This embedding highlights bilinear forms as building blocks in higher multilinear algebra.^[40] In infinite-dimensional settings, such as Banach spaces, care is required with tensor products. The algebraic tensor product V \otimes W exists but is generally not complete with respect to natural norms, so continuous bilinear forms may require completed tensor products (e.g., projective or injective) to ensure the isomorphisms hold in the topological category. For Hilbert spaces, no single completion satisfies the universal property for all continuous bilinear maps to Hilbert spaces.^[42]

Generalizations

Distinct vector spaces

A bilinear form on distinct vector spaces V and W over a field F is a function B: V \times W \to F that is linear in each argument separately.^[43] Specifically, for all v_1, v_2 \in V, w \in W, and \lambda \in F, B(v_1 + v_2, w) = B(v_1, w) + B(v_2, w) and B(\lambda v_1, w) = \lambda B(v_1, w), and similarly for linearity in the second argument.^[43] The set \mathrm{Bil}(V, W) of all such bilinear forms forms a vector space over F, which is naturally isomorphic to the tensor product V^* \otimes W^*, where V^* and W^* denote the dual spaces.^[44] Unlike the case where V = W, bilinear forms on distinct spaces exhibit asymmetries in their non-degeneracy properties. A bilinear form B is left non-degenerate if the induced map V \to W^*, defined by v \mapsto (w \mapsto B(v, w)), is injective, meaning that if B(v, w) = 0 for all w \in W, then v = 0.^[45] Similarly, B is right non-degenerate if the map W \to V^*, w \mapsto (v \mapsto B(v, w)), is injective.^[45] In the non-square case, left non-degeneracy does not imply right non-degeneracy, and vice versa, particularly when \dim V \neq \dim W or in infinite dimensions.^[45] However, if both conditions hold and V, W are finite-dimensional with equal dimension, then B induces an isomorphism V \cong W^*.^[44] In linear algebra, bilinear forms between distinct spaces arise in applications such as pairing linear maps. For instance, consider V = \mathrm{Hom}(U, X) and W = \mathrm{Hom}(X, U) for vector spaces U and X; the trace form B(f, g) = \mathrm{Tr}(f \circ g) defines a bilinear map V \times W \to F, which is non-degenerate under suitable conditions on dimensions.^[44] A fundamental example is the canonical pairing between a vector space and its dual, given by the evaluation map \langle \phi, v \rangle = \phi(v) for \phi \in V^* and v \in V. This bilinear form B: V^* \times V \to F is non-degenerate in both directions when V is finite-dimensional, establishing a natural duality.^[44]

Module extensions

In the more general setting of modules over a commutative ring R, a bilinear form on R-modules M and N is an R-bilinear map B: M \times N \to R, meaning B is additive in each argument and satisfies B(rm, n) = r B(m, n) = B(m, rn) for all r \in R, m \in M, and n \in N.^[46] This extends the vector space case by incorporating the ring structure, where linearity holds with respect to the module actions rather than field scalar multiplication.^[1] Unlike over fields, where the dual module M^\vee = \Hom_R(M, R) often allows straightforward isomorphisms via non-degenerate forms, no such general duality isomorphism exists for arbitrary modules over rings, complicating the identification of bilinear forms with elements of M^\otimes N.^[1] Non-degeneracy, defined as the associated maps M \to N^\vee and N \to M^\vee (given by m \mapsto (n \mapsto B(m,n)) and similarly) being isomorphisms, becomes harder to characterize due to torsion elements; for instance, in modules with 2-torsion, skew-symmetric forms may fail to be alternating, affecting properties like the dimension of isotropic subspaces.^[46]^[1] For free modules such as M = N = R^n, bilinear forms reduce to those represented by n \times n matrices over R, with the standard basis yielding B(e_i, e_j) = a_{ij} for the matrix (a_{ij}).^[1] On abelian groups, viewed as \mathbb{Z}-modules, examples include the form B: \mathbb{Q} \times \mathbb{Q} \to \mathbb{Q} given by B(r, s) = rs. For torsion groups, the tensor product \mathbb{Z}/n\mathbb{Z} \otimes_\mathbb{Z} \mathbb{Z}/m\mathbb{Z} = 0 if \gcd(n,m)=1, illustrating issues with torsion in module tensor products, which bilinear forms help study.^[46] Sesquilinear forms extend this framework over rings A with involution \sigma: A \to A, defined as biadditive maps s: V \times V \to A on a right A-module V satisfying s(xa, yb) = \sigma(a) s(x, y) b, making s linear in the second argument and anti-linear (via \sigma) in the first; this generalizes Hermitian forms over complexes, where \sigma is complex conjugation.^[47] In algebraic geometry, bilinear forms appear on coherent sheaves over varieties, such as non-degenerate symmetric forms on Ulrich sheaves on a projective variety X \subset \mathbb{P}^d, which correspond to real symmetric determinantal representations of X and link to properties like hyperbolicity.^[48]

References

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4. [Q(v + w) − Q(v − w)] defines a bilinear (necessarily symmetric) form on V . A quadratic space is a pair (V,Q) consisting of a vector space V and a ...Missing: mathematics | Show results with:mathematics<|control11|><|separator|>
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