symmetric monoidal (∞,1)-category of spectra
With braiding
With duals for objects
category with duals (list of them)
dualizable object (what they have)
ribbon category, a.k.a. tortile category
With duals for morphisms
monoidal dagger-category?
With traces
Closed structure
Special sorts of products
Semisimplicity
Morphisms
Internal monoids
Examples
Theorems
In higher category theory
The symmetric algebra $S V$ of a vector space is the free commutative algebra over $V$.
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).
We begin with the construction for vector spaces and then sketch how to generalize it.
Suppose $V$ is a vector space over a field $K$. Then the symmetric algebra $S V$ is generated by the elements of $V$ using these operations:
subject to these identities:
It then follows that $S V$ is a graded algebra where $S^p V$ is spanned by $p$-fold products, that is, elements of the form
where $v_1, \dots, v_p \in V$. Clearly $S V$ is also commutative.
The symmetric algebra of $V$ is also denoted $Sym V$. It is also called the polynomial algebra. However we should carefully distinguish between polynomials in the elements of $V$, which form the algebra $S V$, and polynomial functions on the vector space $V$, which form the algebra $S(V^*)$. In quantum physics, a similar construction for Hilbert spaces is known as the Fock space.
More generally, suppose $C$ is any symmetric monoidal category and $V \in C$ is any object. Then we can form the tensor powers $V^{\otimes n}$. If $C$ has countable coproducts we can form the coproduct
(which we write here as a direct sum), and if the tensor product distributes over these coproducts, $T V$ becomes a monoid object in $C$, with multiplication given by the obvious maps
This monoid object is called the tensor algebra of $V$.
The symmetric group $S_n$ acts on $V^{\otimes n}$, and if $C$ is a linear category over a field of characteristic zero, then we can form the symmetrization map
given by
This is an idempotent, so if idempotents split in $C$ we can form its cokernel, called the $n$th symmetric tensor power or symmetric power $S^n V$. The coproduct
becomes a monoid object called the symmetric algebra of $V$.
If $C$ is a more general sort of symmetric monoidal category, then we need a different construction of $S^n V$. For example, if $C$ is a symmetric monoidal category with finite colimits, we can simply define $S^n V$ to be the coequalizer of the action of the symmetric group $S_n$ on $V^{\otimes n}$. And if $C$ also has countable coproducts, we can define
Then, if the tensor product distributes over these colimits (as in a 2-rig), $S V$ will become a commutative monoid object in $C$. In fact, it will be the free commutative monoid object on $V$, meaning that any morphism in $C$
where $A$ is a commutative monoid, factors uniquely as the obvious morphism
followed by a morphism of commutative monoids
as in this commutative triangle:
If $V$ is an object in a CMon-enriched symmetric monoidal category $\mathcal{C}$ such that the following diagram admits a joint coequalizer for every $n \ge 2$:
(there is one morphism for every $\sigma \in \mathfrak{S}_{n}$, such permutation natural transformations being defined in the entry symmetric monoidal category),
we say that $V$ possess all the symmetric powers. The $n^{th}$ symmetric power is given by the coequalizer:
We put $S_{0}(V)=I$ and $S_{1}(V)=V$.
If the category possess the countable coproducts, then we can form
As above, if the tensor product distributes over the countable coproducts and preserve the finite colimits in each argument, then $S(V)$ will be the free commutative monoid object on V.
For example, in the category Rel of sets and relations, this construction gives the set $\mathcal{M}(V)$ of multisets with elements in the set $V$.
(symmetric algebra in chain complexes is differential graded-commutative algebra)
Let the ambient category be the category of cochain complexes over a ground field of characteristic zero, regarded as a symmetric monoidal category via the tensor product of chain complexes.
Then for $(V^\bullet, d)$ a cochain complex, the symmetric algebra
is the differential graded-commutative algebra whose underlying graded algebra is the graded-commutative algebra on $V^\bullet$, and whose differential is the original $d$, extended, uniquely, as a graded derivation of degree +1.
Let the ambient category be the category of cochain complexes over a ground field of characteristic zero, regarded as a symmetric monoidal category via the tensor product of chain complexes and consider the differential graded-commutative algebra $Sym(V^\bullet,d)$ free on a cochain complex $(V^\bullet,d)$ from example .
Then the cochain cohomology (of the underlying cochain complex) of $Sym(V^\bullet,d)$ is the graded symmetric algebra on the cochain cohomology of $(V^\bullet,d)$:
See also this MO discussion.
We have the following sequence of linear isomorphisms:
Here:
the first step uses that, while a priori the symmetric algebra is equivalently the quotient of the tensor algebra by the symmetric group action, in characteristic zero this is equivalently invariants of the symmetric group action, because here $V^G \to V \to V_G$ is a linear isomorphism;
the second step uses that cochain cohomology respects direct sums;
the third step uses that for finite groups in characteristic zero, taking invariants is compatible with passing to cochain cohomology (this prop.):
the fourth step is the Künneth theorem for ordinary homology over a field (this prop.).
Symmetric powers in a symmetric monoidal $\mathbb{Q}^{+}$-linear category are characterized among the countable families of objects as forming a special connected graded quasi-bialgebra (reference to come).
Bourbaki, Algèbre, chap. III, § 6.
On the symmetric algebra spectrum of the sphere spectrum, and its structure as a Hopf ring spectrum:
Further on symmetric powers in homotopical/higher algebra:
Last revised on April 26, 2023 at 06:41:49. See the history of this page for a list of all contributions to it.