Let (κn)∞n=1be i.i.d. copies ofκand let (gn, Un)∞n=1be an independent Poisson point process on G×R with intensitye−uµ(dg)du. We call a process of the form
η(t) :=
∞
_
n=1
Un+κn(gn−1t) t∈T.
Mixed Moving Maxima process (M3 process) w.r.t. the G-action.
For a non-empty finite setM ⊂T the f.d.d. ofη in Definition 5.2.1 are given by
−logP(η(t)≤yt, t∈M) =Eκ
Z
G
exp _
t∈M
κ(g−1t)−yt
! µ(dg)
for y ∈ RM. In particular η is max-stable with standard Gumbel marginals. To see the G-invariance of the f.d.d. of η, fix some y ∈ Rm and locations t1, . . . , tm and note that the function ˜κt1,...,tm(g) := Eκexp Wt∈M κ(g−1ti)−yi
on G is integrable. Sinceµis a (left) Haar measure, we have for anyh∈G
Z
G
˜κht1,...,htm(g)µ(dg) = Z
G
˜κt1,...,tm(h−1g)µ(dg) = Z
G
˜κt1,...,tm(g)µ(dg), which showsP(η(hti)≤yi, i= 1, . . . , m) =P(η(ti)≤yi, i= 1, . . . , m) for allh∈G.
5.3. M3 representation for actions of compact groups
The following theorem is the main result of this chapter. It can be viewed as an analogous version of [52, Theorem 14] for compact groups.
Theorem 5.3.1. Let G be a compact metric group acting continuously on a σ-locally-compact metric spaceT, such that there exists to with
Stab(to)⊂Stab(t) ∀t∈T. (5.4)
Let {W(t)}t∈T be a sample-continuous zero mean Gaussian process with variance σ2(t) = Var(W(t)) and variogram γ(s, t) =E(W(s)−W(t))2, such that
γ(gs, gt) =γ(s, t) ∀s, t∈T, ∀g∈G, (5.5) γ(s, t)6= 0 ∀s, t∈T with s6=t. (5.6) Set ξ(t) := W(t)−σ2(t)/2 and let η be the Brown-Resnick process from Theorem 5.1.2. Then there exists a Mixed Moving Maxima process η∗ that has the same law as η.
Example 5.3.2. The requirement that there exists to with Stab(to) ⊂Stab(t) for all t ∈ T will be met if all stabilizer subgroups Stab(t) ⊂ G for t ∈ T coincide.
For instance, there is an S3 action on SO(3) with stabilizers {±1}. The following scenarios are also included in this situation:
(a) Gacts freely onT, e.g.S1acts freely onR2\{o}by rotation around the origin.
(a.1) Each group G acts freely on itself by left multiplication. For instance, the 3-sphere S3⊂R4 admits a group action and acts freely on itself.
(b) Gacts transitively onT and the stablilizer subgroup Stab(to)⊂Gof someto∈ T (hence every to ∈T) is a normal subgroup of G (meaning gStab(to)g−1 = Stab(to) for g∈ G). In this case T ∼=G/Stab(to) is necessarily compact (see Lemma 5.3.4).
(b.1) This is includes transitive actions of abelian groups, e.g. S1 ×S1 acts transitively on the 2-dimensional torus.
On the other hand, it is not necessary, to require that all stabilizer subgroups coincide. For instance, in addition to the above scenarios fixed points t∗ (with Stab(t∗) =G) can be admitted as in the rotatingS1-action onR2or on the 2-sphere S2. However, (5.4) does not include the case that the stabilizer subgroups are only conjugate. The isometric SO(3)-action onS2 may not be considered, for example.
Example 5.3.3. Consider again the rotatingS1-action on the sphereS2, which has two fixed points. This example meets (5.4). Many suitable variograms for Theorem 5.3.1 that depend only on the spherical distance can be found in [43].
The following lemma summarizes some facts concerning the group action involved in Theorem 5.3.1. We refer to [73] and [72]. A detailed study of Haar measures on locally compact homogeneous spaces can be found in [68].
Lemma 5.3.4. Let Gbe a compact metric group acting continuously on a σ-locally-compact metric space T (from the left). Let to∈T and Gto ⊂T its (compact) orbit andStab(to)⊂G the stabilizer of to, which is a closed (hence compact) subgroup of G. Then the map
G/Stab(to)→Gto, [g]→gto
is well-defined and a homeomorphism. In particular Ho := G/Stab(to) ∼= Gto is compact and locally compact. The groupG acts continuously onHo by left multipli-cation: g[h] := [gh].
5.3. M3 representation for actions of compact groups 111
If additionally Stab(to) is a normal subgroup in G, then Ho is also a compact metric group and the continuous quotient map G → Ho where g 7→ [g] is a group homomorphism. Any finite G-invariant (or equivalently Ho-invariant) measure on the Borel-σ-algebra of Ho is a constant multiple of the (left) Haar measure µ˜ on Ho with µ(H˜ o) = 1. The (probability) Haar measure µ˜ on Ho is the pushforward measure of the (probability) Haar measureµonGunder the quotient mapG→Ho. Before we can prove Theorem 5.3.1, we need to establish a crucial preparatory result, which is also of independent interest.
Lemma 5.3.5. Let (T, d) be a compact metric space and (C(T),k·k) the Banach space of real-valued continuous functions on T endowed with the supremum norm k·k. Let Camax(T) be the subset of functions f ∈ C(T), such that there exists a unique point t∈ T, at which the maximum value of f is attained. Then Camax(T) is measurable w.r.t. the Borel-σ-algebra of the supremum norm and the map
argmax :Camax(T)→T,
which assigns to each function f ∈Camax(T) this unique point, is well-defined and measurable.
Proof. Forf ∈C(T) we define the set of its maximizers set- argmax(f) :=
t∈T : f(t) = max
t∈T f(t)
,
which is a non-empty compact subset ofT, since T is compact andf is continuous.
Thus, we have a well-defined map set-argmax : C(T) → K(T) into K(T), the set of non-empty compact subsets ofT. By choosing a countable dense subset S of the compact metrizable spaceT, we can express set-argmax(f) as
set-argmax(f) = \
s∈S
{t∈T : f(t)≥f(s)},
sincef is continuous. Now, for eachs∈S, the setAs(f) :={t∈T : f(t)≥f(s)}is a non-empty compact subset ofT, sinceT is compact andf is continuous. Moreover the assignment C(T) → K(T) with f 7→ As(f) is measurable with respect to the Borel-σ-algebra of the Fell topology on K(T). To see this, e.g. write As(f) = {t ∈ T : fs(t) ≥ 0} as upper level set of the continuous function fs(t) = f(t)− f(s), which depends continuously on f ∈ C(T) (cf. [65, Example 1.2]). Hence, also the countable intersection set-argmax(f) = Ts∈SAs(f) : C(T) → K(T) is measurable with respect to the Borel-σ-algebra of the Fell topology on K(T) (cf.
[65, Theorem 2.25]). Because T is compact, the Fell topology on K(T) coincides with the topology of the Hausdorff distanceρdwith respect to the metricdonT (cf.
[3, Theorem 3.93]). Thus, set-argmax :C(T)→K(T) is measurable with respect to the Borel-σ-algebra of the Hausdorff distanceρd. Further, the map
i:T →K(T), i(t) ={t}
embeds (T, d) isometrically intoK(T) as a closed (hence measurable) subset ofK(T) (cf. [3, Lemma 3.78]). We may conclude that
Camax(T) = set-argmax−1(i(T))
is measurable (with respect to the Borel-σ-algebra of the supremum norm). More-over, the map
argmax :=i−1◦set-argmax :Camax(T)→T
is well-defined. Let A ⊂ T be closed (and hence compact). Then i(A) ⊂ K(T) is also compact, since iis continuous. Therefore,
argmax−1(A) = set-argmax−1(i(A))⊂Camax(T) is measurable, which shows that the map argmax is measurable.
Proof of Theorem 5.3.1. We take the proof of Theorem 14 in [52] as a guideline adding some subtle changes. Let (Ω,A,PW) denote the probability space corre-sponding to the Gaussian process W : Ω →C(T). The processξ : Ω →C(T) with ξ(t) :=W(t)−σ2(t)/2 is also Gaussian and continuous. A short computation shows that
Var(ξ(s)−ξ(t)) =γ(s, t) (5.7)
Because of (5.4), we have that Stab(to) ⊂ Stab(gto) = gStab(to)g−1 for all g ∈ G.
Hence, Stab(to)⊂Gis a normal subgroup ofG. By Lemma 5.3.4, we have that the map G/Stab(to) → Gto given by [g] 7→ gto is well-defined and a homeomorphism.
In particular
Ho :=G/Stab(to)∼=Gto
is a compact metric group and G acts continuously on Ho by left multiplication:
5.3. M3 representation for actions of compact groups 113
g[h] := [gh]. Moreover, the following map is well-defined and continuous
()o:C(T)→C(Ho), f 7→fo, wherefo([g]) :=f(gto). (5.8) Applying this map ()o to ξ yields a sample continuous Gaussian process ξo : Ω → C(Ho). Since Ho is compact metric, each sample path of ξo attains its maximum and the set Camax(Ho) of continuous functions on Ho with unique maximizer is measurable, cf. Lemma 5.3.5. Further, due to (5.7) and (5.6) we have
Var(ξo([g1])−ξo([g2])) =γ(g1to, g2to)6= 0 ∀[g1]6= [g2].
Thus, [53, Lemma 2.6.] ensures that PW(ξo−1(Camax(Ho))) = 1. So, the Gaussian processW allows for an undistinguishable processW∗on Ω by replacing the sample paths on the complement of ξ−1o (Camax(Ho)) in Ω by some continuous path, such that the process ξo∗([g]) :=ξ∗(gto) :=W∗(gto)−σ2(gto)/2 attains its maximum at only one point. We denote the preimage of Camax(Ho) under the continuous map (5.8) by Camax,o(T). Then,ξ∗ takes its values only inCamax,o(T)⊂C(T).
In what follows, we will show that the corresponding Brown-Resnick process η∗ defined through Theorem 5.1.2 (which has the same law as η) has a Mixed Mov-ing Maxima representation. For notational convenience we will omit the index ()∗ henceforth. From now on, we can stick closer to the proof of Theorem 14 in [52].
Firstly, Lemma 5.3.5 ensures that the argmax-map argmax :Camax(Ho) →Ho is measurable. This is the crucial point when verifying that the following maps are measurable:
R×Camax,o(T) π //
pQQQQQQ((
QQ QQ
QQ Ho×R×C(T)
Camax,o(T)
Π
66m
mm mm mm mm mm m
π(u, ξ) := ([g0], y0, f0), where [g0] := argmax(ξo), y0 :=u+ max(ξo), f0(t) :=u+ξ(g0t)−y0 Π(f) := ([g00], y00, f00), where [g00] := argmax(fo),
y00:= max(fo), f00(t) :=f(g00t)−y00 p(u, ξ) :=u+ξ
Note that p is well-defined and that the continuous functions f0 ∈ C(T) and f00∈C(T) are only well-defined since Stab(to)⊂Stab(t) for all t∈T (as assumed, see (5.4)). The above diagram commutes, i.e. π= Π◦p and
Π(g∗f) = (g−1[g00], y00, f00) ∀g∈G, (5.9) whereg∗f denotes the function on T given by g∗f(t) =f(gt), which belongs again toCamax,o(T).
Now, the processηis constructed by (5.1) from a Poisson point process (Un, ξn)∞n=1 on R× C(T) with intensity measure e−udu× dPξ where Pξ is the law of ξ on Camax,o(T). This process transforms under π to a Poisson point process on Ho× T×C(T) with intensity measure
Ψ(A) = Z
π−1(A)
e−udu×dPξ= Z
R
e−uP[([g0], y0, f0)∈A] du
for sets A in the Borel-σ-algebra of Ho×T ×C(T), cf. the mapping theorem for Poisson point processes [54]. By the same computation as in [52] it follows that
Ψ({([h], y+z, f) : ([h], y, f)∈A}) =e−zΨ(A) ∀z∈R. (5.10) Because γ is G-invariant (see (5.2)) and π factorizes by the above diagram and Π commutes with theG action in the sense of (5.9), Theorem 1 in [49] implies that
Ψ({(g−1[h], y, f) : ([h], y, f)∈A}) = Ψ(A) ∀g∈G. (5.11) Further, we have that
Ψ(Ho×[0,1]×C(T)) = Z
R
e−uP(y0 ∈[0,1]) du= Z
R
e−uP(u+ max(ξo)∈[0,1]) du
≤ Z
R
evP(max(ξo)≥v) dv≤ Z
R
evP(kξokC(Ho) ≥v) dv <∞ where the last inequality follows from the asymptotics of Corollary 3.2 in [56].
Following [52] we introduce the measure ΨA on Ho×R for a measurable subset A ⊂ C(T) via ΨA(B) = RB×AeydΨ([g], y, f). Because of (5.11) the measure ΨA is G-invariant (or equivalentlyHo-invariant) in the first componentHo and due to (5.10) it is translation invariant in the second componentR. Hence ΨAis a multiple of the product measure which is built from the (probability) Haar measure ˜µon Ho
(see Lemma 5.3.4) and the Lebesgue measure on R. Therefore, we write dΨA = Q(A)˜µ(d[g])dy for some positive constant Q(A), where the assignment A 7→ Q(A)