Positive-definite function on a group
# Positive-definite function on a group

In mathematics, and specifically in operator theory, a positive-definite function on a group relates the notions of positivity, in the context of Hilbert spaces, and algebraic groups. It can be viewed as a particular type of positive-definite kernel where the underlying set has the additional group structure.

## Definition

Let G be a group, H be a complex Hilbert space, and L(H) be the bounded operators on H. A positive-definite function on G is a function F: G → L(H) that satisfies

    $\sum _{s,t\in G}\langle F(s^{-1}t)h(t),h(s)\rangle \geq 0,$

for every function h: G → H with finite support (h takes non-zero values for only finitely many s).

In other words, a function F: G → L(H) is said to be a positive-definite function if the kernel K: G × G → L(H) defined by K(s, t) = $F(s^{−1}t)$ is a positive-definite kernel.

## Unitary representations

A unitary representation is a unital homomorphism Φ: G → L(H) where Φ(s) is a unitary operator for all s. For such Φ, $Φ(s^{−1})$ = Φ(s)*.

Positive-definite functions on G are intimately related to unitary representations of G. Every unitary representation of G gives rise to a family of positive-definite functions. Conversely, given a positive-definite function, one can define a unitary representation of G in a natural way.

Let Φ: G → L(H) be a unitary representation of G. If P ∈ L(H) is the projection onto a closed subspace H` of H. Then F(s) = P Φ(s) is a positive-definite function on G with values in L(H`). This can be shown readily:

    ${\begin{aligned}\sum _{s,t\in G}\langle F(s^{-1}t)h(t),h(s)\rangle &=\sum _{s,t\in G}\langle P\Phi (s^{-1}t)h(t),h(s)\rangle \\&=\sum _{s,t\in G}\langle \Phi (t)h(t),\Phi (s)h(s)\rangle \\&=\left\langle \sum _{t\in G}\Phi (t)h(t),\sum _{s\in G}\Phi (s)h(s)\right\rangle \\&\geq 0\end{aligned}}$

for every h: G → H` with finite support. If G has a topology and Φ is weakly(resp. strongly) continuous, then clearly so is F.

On the other hand, consider now a positive-definite function F on G. A unitary representation of G can be obtained as follows. Let $C_{00}(G,$ H) be the family of functions h: G → H with finite support. The corresponding positive kernel K(s, t) = $F(s^{−1}t)$ defines a (possibly degenerate) inner product on $C_{00}(G,$ H). Let the resulting Hilbert space be denoted by V.

We notice that the "matrix elements" K(s, t) = $K(a^{−1}s,$ $a^{−1}t)$ for all a, s, t in G. So $U_ah(s)$ = $h(a^{−1}s)$ preserves the inner product on V, i.e. it is unitary in L(V). It is clear that the map Φ(a) = $U_a$ is a representation of G on V.

The unitary representation is unique, up to Hilbert space isomorphism, provided the following minimality condition holds:

    $V=\bigvee _{s\in G}\Phi (s)H\,$

where $\bigvee$ denotes the closure of the linear span.

Identify H as elements (possibly equivalence classes) in V, whose support consists of the identity element e ∈ G, and let P be the projection onto this subspace. Then we have $PU_aP$ = F(a) for all a ∈ G.

## Toeplitz kernels

Let G be the additive group of integers Z. The kernel K(n, m) = F(m − n) is called a kernel of Toeplitz type, by analogy with Toeplitz matrices. If F is of the form F(n) = $T^n$ where T is a bounded operator acting on some Hilbert space. One can show that the kernel K(n, m) is positive if and only if T is a contraction. By the discussion from the previous section, we have a unitary representation of Z, Φ(n) = $U^n$ for a unitary operator U. Moreover, the property $PU_aP$ = F(a) now translates to $PU^nP$ = $T^n.$ This is precisely Sz.-Nagy's dilation theorem and hints at an important dilation-theoretic characterization of positivity that leads to a parametrization of arbitrary positive-definite kernels.

