nLab
symmetric algebra

Context

Algebra

Monoidal categories

Contents

Idea

The symmetric algebra SVS V of a vector space is the free commutative algebra over VV.

This construction generalizes to group representations, chain complexes, vector bundles, coherent sheaves, and indeed objects in any symmetric monoidal linear categories with enough colimits, where the tensor product distributes over those colimits (as in a 2-rig).

Explicit definition

We begin with the construction for vector spaces and then sketch how to generalize it.

For vector spaces

Suppose VV is a vector space over a field KK. Then the symmetric algebra SVS V is generated by the elements of VV using these operations:

  • addition and scalar multiplication
  • an associative binary operation \cdot

subject to these identities:

  • the identities necessary for SVS V to be an associative algebra
  • the identity vw=wvv \cdot w = w \cdot v for all vVv \in V.

It then follows that SVS V is a graded algebra where S pVS^p V is spanned by pp-fold products, that is, elements of the form

v 1v pv_1 \cdot \cdots \cdot v_p

where v 1,,v pVv_1, \dots, v_p \in V. Clearly SVS V is also commutative.

The symmetric algebra of VV is also denoted SymVSym V. It is also called the polynomial algebra. However we should carefully distinguish between polynomials in the elements of VV, which form the algebra SVS V, and polynomial functions on the vector space VV, which form the algebra S(V *)S(V^*). In quantum physics, a similar construction for Hilbert spaces is known as the Fock space.

In general

More generally, suppose CC is any symmetric monoidal category and VCV \in C is any object. Then we can form the tensor powers V nV^{\otimes n}. If CC has countable coproducts we can form the coproduct

TV= n0V n T V = \bigoplus_{n \ge 0} V^{\otimes n}

(which we write here as a direct sum), and if the tensor product distributes over these coproducts, TVT V becomes a monoid object in CC, with multiplication given by the obvious maps

V pV qV (p+q) V^{\otimes p} \otimes V^{\otimes q} \to V^{\otimes (p+q)}

This monoid object is called the tensor algebra of VV.

The symmetric group S nS_n acts on V nV^{\otimes n}, and if CC is a linear category over a field of characteristic zero, then we can form the symmetrization map

p A:V nV n p_A : V^{\otimes n} \to V^{\otimes n}

given by

p A=1n! σS nσ p_A = \frac{1}{n!} \sum_{\sigma \in S_n} \sigma

This is an idempotent, so if idempotents split in CC we can form its cokernel, called the nnth symmetric tensor power or symmetric power S nVS^n V. The coproduct

SV= n0S nV S V = \bigoplus_{n \ge 0} S^n V

becomes a monoid object called the symmetric algebra of VV.

If CC is a more general sort of symmetric monoidal category, then we need a different construction of S nVS^n V. For example, if CC is a symmetric monoidal category with finite colimits, we can simply define S nVS^n V to be the coequalizer of the action of the symmetric group S nS_n on V nV^{\otimes n}. And if CC also has countable coproducts, we can define

SV= n0S nV S V = \coprod_{n \ge 0} S^n V

Then, if the tensor product distributes over these colimits (as in a 2-rig), SVS V will become a commutative monoid object in CC. In fact, it will be the free commutative monoid object on VV, meaning that any morphism in CC

VA, V \to A \, ,

where AA is a commutative monoid, factors uniquely as the obvious morphism

VSV V \to S V

followed by a morphism of commutative monoids

SVA, S V \to A \, ,

as in this commutative triangle:

SV V A \array { & & S V \\ & \nearrow & & \searrow \\ V & & \longrightarrow & & A }

Revised on February 19, 2013 01:05:04 by Urs Schreiber (80.81.16.253)