Ergodic properties of functionals of Gaussian processes
Pith reviewed 2026-05-10 07:13 UTC · model grok-4.3
The pith
Positive functionals of stationary Gaussian processes have their first two moments determined exactly by the one- and two-time probability densities.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We derive the first two moments of generic positive stochastic functionals in terms of the one- and two-time probability density functions of the underlying random walk, and we prove ergodicity of observables in stationary random walks. These general results are applied to the half-occupation time and the occupation time in an interval of a Gaussian random walk, for which we obtain exact analytic expressions for the first two moments. We then extend the analysis to scaled Brownian motion and fractional Brownian motion, computing the ergodicity breaking parameter and establishing a simple scaling form for the probability densities of occupation times. Within the framework of infinite ergodic
What carries the argument
Reduction of the moments of a positive functional to integrals over the one- and two-time probability densities of the underlying stationary process.
If this is right
- Exact analytic expressions exist for the first two moments of half-occupation time and interval occupation time in Gaussian random walks.
- An ergodicity breaking parameter can be computed for scaled Brownian motion and fractional Brownian motion.
- Probability densities of occupation times obey a simple scaling form in these processes.
- Positive observables share universal properties under infinite ergodic theory.
Where Pith is reading between the lines
- The same reduction may apply to other positive path functionals such as the time integral of a nonlinear function of the position.
- Relaxing stationarity would require time-dependent densities and would likely produce non-ergodic behavior whose parameter dependence remains to be mapped.
- The scaling forms found for fractional Brownian motion suggest analogous reductions could be tested in other anomalous-diffusion models that admit two-time densities.
Load-bearing premise
The moments of the positive functionals are fully determined by the one- and two-time probability densities when the underlying process is a stationary Gaussian random walk or a related fractional Brownian motion.
What would settle it
A direct Monte Carlo sampling of the first two moments of the half-occupation time for a stationary Gaussian walk that deviates from the closed-form expressions obtained by integrating the one- and two-time densities.
Figures
read the original abstract
We derive the first two moments of generic positive stochastic functionals in terms of the one- and two-time probability density functions of the underlying random walk, and we prove ergodicity of observables in stationary random walks. These general results are applied to the half-occupation time and the occupation time in an interval of a Gaussian random walk, for which we obtain exact analytic expressions for the first two moments. We then extend the analysis to scaled Brownian motion and fractional Brownian motion, computing the ergodicity breaking parameter and establishing a simple scaling form for the probability densities of occupation times. Within the framework of infinite ergodic theory, we further identify universal properties of positive observables. All analytical predictions are fully confirmed by numerical simulations.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript derives the first two moments of generic positive stochastic functionals of random walks in terms of the one- and two-time probability density functions of the underlying process. It proves ergodicity of observables for stationary random walks and applies the framework to the half-occupation time and occupation time in an interval for Gaussian random walks, obtaining exact analytic expressions for the moments. The analysis is extended to scaled Brownian motion and fractional Brownian motion, yielding the ergodicity breaking parameter, a scaling form for the occupation-time PDFs, and universal properties identified via infinite ergodic theory. All analytic predictions are stated to be confirmed by numerical simulations.
Significance. If the derivations are correct, the work supplies explicit analytic results for moments of occupation functionals in Gaussian and self-similar processes, which are relevant for quantifying ergodicity breaking in anomalous diffusion. The general moment expressions and the infinite-ergodic-theory discussion provide a systematic route to positive observables, while the numerical agreement supports the closed-form claims for the specific cases treated.
major comments (2)
- [general derivation (prior to specific applications)] The general derivation of the first two moments from one- and two-time PDFs follows directly from the integral definitions of the expectations; the manuscript should clarify the novel technical step beyond this identity, especially since the ergodicity statement for stationary walks follows from the standard Birkhoff ergodic theorem.
- [section on fBM and infinite ergodic theory] For the extension to fractional Brownian motion and infinite ergodic theory, the universal properties of positive observables require an explicit statement of the invariant measure and the class of observables for which the scaling form of the PDFs holds; without this, the claim that the properties are universal remains incompletely supported.
minor comments (3)
- The numerical simulations confirming the analytic expressions should report the number of trajectories, discretization scheme, and statistical error estimates to permit independent verification.
- Notation for the one- and two-time PDFs should be introduced consistently and distinguished from the occupation-time densities to avoid ambiguity in the integral expressions.
- A brief comparison with existing results on occupation times for Brownian motion (e.g., Lévy arcsine law and its generalizations) would help situate the new analytic expressions.
Simulated Author's Rebuttal
We thank the referee for the careful reading of our manuscript and the constructive comments. We appreciate the positive assessment of the work's significance and the recommendation for minor revision. Below we provide point-by-point responses to the major comments, indicating the revisions we will implement.
read point-by-point responses
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Referee: [general derivation (prior to specific applications)] The general derivation of the first two moments from one- and two-time PDFs follows directly from the integral definitions of the expectations; the manuscript should clarify the novel technical step beyond this identity, especially since the ergodicity statement for stationary walks follows from the standard Birkhoff ergodic theorem.
Authors: We thank the referee for highlighting this point. The moment expressions do follow from direct integration, but the technical contribution of the general framework is to express the first two moments of arbitrary positive functionals solely in terms of the one- and two-time PDFs of the underlying process. This formulation is what subsequently permits closed-form analytic results for the specific occupation-time functionals without invoking higher-order statistics. For the ergodicity statement, we apply the Birkhoff theorem to stationary walks but explicitly verify its consequences for the positive observables under consideration. We will revise the general derivation section to clarify these aspects and distinguish the identity from the subsequent applications. revision: partial
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Referee: [section on fBM and infinite ergodic theory] For the extension to fractional Brownian motion and infinite ergodic theory, the universal properties of positive observables require an explicit statement of the invariant measure and the class of observables for which the scaling form of the PDFs holds; without this, the claim that the properties are universal remains incompletely supported.
Authors: We agree that an explicit statement strengthens the universality claim. Within the infinite ergodic theory framework applied here, the relevant object is the infinite invariant measure associated with the null-recurrent dynamics of the underlying Gaussian process, and the scaling form of the occupation-time PDFs holds for the class of positive observables that possess a finite mean with respect to this measure. We will add a concise paragraph in the fractional Brownian motion section stating the invariant measure (with reference to the relevant infinite ergodic theory results) and the precise conditions on the observables, thereby supporting the universality statement. revision: yes
Circularity Check
No significant circularity; derivation is definitional and self-contained
full rationale
The central results express the first two moments of positive functionals directly as integrals over the one- and two-time PDFs of the underlying process. This follows immediately from the definition of expectation for any stochastic process and does not constitute a non-trivial derivation that reduces to its own inputs. For the Gaussian and fBM cases the bivariate densities are known in closed form, permitting explicit integration without parameter fitting or self-referential steps. Ergodicity claims for stationary walks invoke standard Birkhoff arguments, while extensions to infinite ergodic theory follow from scaling already present in the moment expressions. No self-citation is load-bearing, no ansatz is smuggled, and no uniqueness theorem is invoked from prior author work. The paper is therefore self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption One- and two-time probability density functions of stationary Gaussian random walks fully determine moments of positive functionals
- domain assumption Results from infinite ergodic theory apply to positive observables of these processes
Reference graph
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Half occupation time We consider here the half occupation timeT +(t) for the SBM. From (52) and (63) T +(t)2 = t2 2 − 1 π Z t 0 dτ2 Z τ2 0 dτ1 arctan s τ α 2 τ α 1 −1 ! = t2 2 " 1− 1 π Z 1 0 arctan r 1 uα −1 ! du # .(64) The integral in the above expression can be solved by introducing the new variabley= (u−α −1) 1/2 Z 1 0 arctan r 1 uα −1 ! du= 2 α Z ∞ 0...
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[2]
Occupation time in an interval 102 103 104 105 106 t 101 102 103 104 Ta(t) a) =0.5 =1.0 =1.5 102 103 104 105 106 t 102 104 106 108 Ta(t)2 b) =0.5 =1.0 =1.5 FIG. 2. First -a)- and second -b)- moments of the occupation time in an interval as a function oftfor the SBM. The symbols are obtained from numerical simulations forα= 0.5,1.0,and 1.5. The solid lines...
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[3]
Half occupation time 102 103 104 105 106 t 0.40 0.45 0.50 0.55 0.60 T + (t) /t a) H=0.2 H=0.4 H=0.6 H=0.8 102 103 104 105 106 t 0.30 0.35 0.40 0.45 0.50 T + (t)2 /t2 b) H=0.2 H=0.4 H=0.6 H=0.8 FIG. 4. First -a)- and second -b)- moments of the half occupation time overtandt 2 respectively as a function oftfor the fBM. The symbols are obtained from numerica...
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[4]
Occupation time in an interval 102 103 104 105 106 t 101 102 103 104 105 Ta(t) a) H=0.2 H=0.4 H=0.6 H=0.8 102 103 104 105 106 t 104 106 108 1010 Ta(t)2 b) H=0.2 H=0.4 H=0.6 H=0.8 FIG. 5. First -a)- and second -b)- moments of the occupation time in an interval as a function of tfor the fBM. The symbols are obtained from numerical simulations forH= 0.2,0.4,...
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(which we include in the plot as orange squares with the label KA), the EB a computed from the ML distribution is valid solely in the region nearH= 1/2, where Eq. (76) and (70) provide similar results. For values ofH→0 andH→1 we find that (70) is no longer valid. However, our result (76) is exact and holds for the whole range of values ofH. 0.2 0.4 0.6 0....
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