Doering Savov

AN APPLICATION OF RENEWAL THEOREMS TO EXPONENTIAL
MOMENTS OF LOCAL TIMES

¨
LEIF DORING AND MLADEN SAVOV

Abstract. In this note we explain two transitions known for moment generating functions of
local times by means of properties of the renewal measure of a related renewal equation. The
arguments simplify and strengthen results on the asymptotic behavior in the literature.
Suppose (Xt ) is a time-homogeneous continuous time Markov process on a countable set S with
transition probabilities pt (i, j) = P[Xt = j |X0 = i] for i, j ∈ S. We fix some arbitrary i ∈ S and
denote by Li the time (Xt ) spends at i until time t:
t
t
Li =
t δi (Xs ) ds.
0
i
A quantity that has been studied in different contexts is the moment generating function Ei eγLt ,
where X0 = i and γ is a positive real number.
i
To explain the interest in Ei eγLt let us have a brief look at the parabolic Anderson model with
Brownian potential, i.e.
(0.1) dut (i) = ∆ut (i) dt + γut (i)dBt (i)
with homogeneous initial conditions u0 ≡ 1. Here, i ∈ Zd , ∆ denotes the discrete Laplacian
∆f (i) = |i−j|=1 1/(2d)(f (j)−f (i)), and {B(i)}i∈Zd is a family of independent Brownian motions.
It is known (see for instance Theorem II.3.2 of [CM94]) that the moments of ut (i) solve discrete-
space heat equations with one-point potentials. In particular, E[ut (i)ut (j)] solves
d
(0.2) w(t, i, j) = ∆w(t, i, j) + γδ0 (i − j)w(t, i, j)
dt
with homogeneous initial conditions. The discrete Laplacian acts on both spatial variables i and
j seperately. Applying the Feynman-Kac formula one reveals that
Rt 1 2
δ0 (Xs −Xs ) ds
w(t, i, j) = Ei,j eγ 0

where X 1 , X 2 are independent simple random walks. Hence, for Lt corresponding to the difference
walk X 1 − X 2 (or equivalently corresponding to a simple random walk with doubled jump rate)
0
E ut (i)2 = w(t, i, i) = E0 eγLt .
The notion of weak 2-intermittency, i.e. exponential growth of the second moment E[ut (i)2 ], now
explains the interest in the study of the exponential moment of Li for continuous time Markov
t
processes.
Applying the variation of constant formula to solutions of (0.2) one can guess that the following
renewal equation holds for fixed γ ≥ 0 and t ≥ 0:
t
i i
(0.3) Ei eγLt = 1 + γ Ei eγLt−s ps (i, i) ds.
0
Indeed, expanding the exponential one can show directly the validity of Equation (0.3) for general
time-homogeneous Markov processes on countable state spaces (see Lemma 3.2 of [AD09]). The
j
same equation holds for Ei eLt with ps (i, i) replaced by ps (i, j). As the analysis does not change

2000 Mathematics Subject Classification. Primary 60J27; Secondary 60J55.
Key words and phrases. Renewal Theorem, Local Times.
The first author was supported by the EPSRC grant EP/E010989/1.
1

2 ¨

we restrict ourselves to i = j.

In Section III of [CM94] and as well in Lemma 1.3 and Theorem 1.4 of [GdH06] analytic tech-
niques were applied to understand the longtime behavior of solutions of (0.2) by means of spectral
properties of the discrete Laplacian with one-point potential. They showed that the exponential
growth rate
1 1 1 0
r(γ) := lim log E ut (i)2 = lim log w(t, i, i) = lim log E0 eγLt .
t→∞ t t→∞ t t→∞ t

exists and obeys the following transition in γ:
r(γ) > 0 if and only if γ > 1/G∞ (i, i),
∞
where G∞ (i, i) is the Green function 0 ps (i, i) ds of the difference random walk. This first
transition in γ can be proved analytically, as identifying r(γ) corresponds to identifying the smallest
eigenvalue of the perturbed operator H = ∆+γδ0 . As multiplication with γδ0 is a one-dimensional
perturbation and for the discrete Laplacian explicit formulas for eigenfunctions are available, all
necessary quantities can be calculated. In particular the exponential growth rate r(γ), in the
general case of part (1) our Theorem 1 represented by the Laplace transform of the transition
probabilities, has been described for the simple random walk as the unique solution of
2 1 1
= ds
γ (2π)d S2 Φ(s) + r(γ)γ/2
d
where S 2 denotes the d-dimensional torus and Φ(s) = 2 i=1 (1 − cos(si )). Compared to this
expression, our Laplace transform representation is particularly useful as it immediately provides
the qualitative behavior of r(γ) as a function of γ.

Replacing the discrete Laplacian by a generator of a finite range random walk, in [DD06] the
second moment of solutions of (0.1) were analyzed via a random walk representation. For more
general initial conditions this leads to a renewal equation similar to (0.3). The authors analyzed
their equation (3.16) directly without appealing to the renewal theorem. In precisely the same
way as we do in the proof of our Theorem 1 one can proceed in their case and strengthen the
asymptotics of their Equation (3.15).

Assuming only that pt (i, i) ∼ ct−α for some α > 0 (by ∼ we denote strong asymptotic equivalence
at infinity) a second transition was revealed in Proposition 3.12 of [AD09] by a Laplace transform
technique combined with Tauberian theorems: at the critical point γ = 1/G∞ the growth is of
linear order if and only if α > 2. As for the simple random walk on Zd the local central limit
theorem implies pt (i, i) ∼ ct−d/2 , linear growth occurs for dimensions at least 5.

The main goal of the following is to show how the known results easily follow from different renewal
theorems utilizing the fact that Equation (0.3) is a renewal equation of the type
t
(0.4) Z(t) = z(t) + Z(t − s)U (ds)
0
i
with Z(t) = Ei eγLt , initial condition z ≡ 1, and renewal measure U (ds) = γps (i, i) ds. This
approach is robust as there is no need to assume any properties of the underlying Markov process
(neither symmetry to obtain a self-adjoint operator, nor polynomial decay for Tauberian theorems
or finite range transitions kernels).
The two transitions will now appear in terms of whether or not the renewal measure
• is a probability measure,
• has a finite mean.
In the supercritical case of γ > 1/G∞ (i, i) without any further consideration we obtain the strong
i
asymptotics of Ei eγLt . This of course is stronger than considering the Lyapunov exponent r(γ)

AN APPLICATION OF RENEWAL THEOREMS TO EXPONENTIAL MOMENTS OF LOCAL TIMES 3

that appears in [CM94], [GdH06], and [AD09] as we exclude the possible existence of a subexpo-
nential factor.
∞
For the statement of the theorem we denote by H∞ (i, i) = 0 sps (i, i) ds the expected time of
hitting of two independent copies of (Xt ). In contrast to the Green function G∞ here we count
the hitting time of the entire paths not only of the paths at same time. The Laplace transform in
time of pt (i, i) is denoted by p(λ), λ > 0, and weak asymptotic equivalence at infinity by f ≈ g.
ˆ
Theorem 1. Suppose (Xt ) is a time-homogeneous Markov process on S started in i. Then for
t
Li = 0 δi (Xs ) ds the following holds:
t
(1) If γ > 1
G∞ (i,i) , then p−1 (1/γ) > 0 and
ˆ
i 1 −1
Ei eγLt ∼ ∞ ep
ˆ (1/γ)t
.
p−1 (1/γ)γ
ˆ 0
se−p−1 (1/γ)s p
ˆ
s (i, i) ds
1
(2) If γ = G∞ (i,i) , then
i t
Ei eγLt ≈ t ∞ ,
γ 0 s
pr (i, i) drds
where the asymptotic bounds are 1 and 2. If moreover
∞
H∞ (i, i) = sps (i, i) ds < ∞,
0
then
i t
Ei eγLt ∼ .
γH∞ (i, i)
1
(3) If 0 ≤ γ < G∞ (i,i) , then
i 1
lim Ei eγLt = .
t→∞ 1 − γG∞ (i, i)
Remark 1. In the general case, we only obtained weak convergence in (2) of the previous theorem
with asymptotic bounds 1 and 2. Under the stronger assumptions pt (i, i) ∼ ct−α , in Proposition
3.12 of [AD09] strong asymptotics were obtained by Tauberian theorems. The case of α > 2 is
contained in the second part of (2), α ≤ 1 is contained in part (1) of our previous theorem and
also strong asymptotics for α ∈ (1, 2] can be obtained by extended renewal theorems. Here, we can
directly use the infinite mean renewal Theorem 1 of [AA87] to obtain precisely the same strong
asymptotics as of Proposition 3.12 of [AD09].
Proof. The proof of Theorem 1 is based on the renewal equation (0.4) setting z ≡ 1, Z(t) =
i
Ei eγLt , and U (ds) = γps (i, i) ds.
(1) The assumptions of the theorem directly imply that in this case U is not a probability measure.
Either the measure is infinite (with density bounded by γ) or it is finite with total mass strictly
larger than 1. From the definition of the Laplace transform we obtain for λ = p−1 (1/γ) that
ˆ
¯
U (ds) = γe−λs ps (i, i) ds
is a probability measure. As by assumption λ > 0, we obtain that e−λt is directly Riemann
¯ ∞
integrable and U has a finite mean γ 0 sps (i, i)e−λs ds. Hence,
t
i i
e−λt Ei eγLt = e−λt + γ e−λ(t−s) Ei eγLt−s e−λs ps (i, i) ds
0
is a proper renewal equation. The classical renewal theorem (see for instance page 363 of [F71])
implies that
∞ −λs
i e ds 1
lim e−λt Ei eγLt = ∞
0
−λs p (i, i) ds
= ∞
t→∞ γ 0
se s λγ 0
se−λs ps (i, i) ds

4 ¨

Figure 1. γ → r(γ) for G∞ (i, i) = ∞ and G∞ (i, i) < ∞ respectively

proving the claim.
∞
(2) In the critical case γ 0 ps (i, i) ds = 1, the measure U as defined above indeed is a probability
measure which does not necessarily has a finite mean. Furthermore, the situation is different from
the first case as now the initial condition z ≡ 1 is not directly Riemann integrable. Iterating
Equation (0.3) we obtain the representation
t
i
Ei eγLt = ps (i, i)∗n ds,
0 n≥0

where ∗n denotes n-fold convolutions. Note that convergence of the series is justified by bounded-
ness of p. In the case of finite mean, Equation (1.2) of page 358 of [F71] and the renewal theorem
on page 360 now directly imply
i t t
Ei eγLt ∼ ∞ = .
γ 0 sps (i, i) ds γH∞ (i, i)
i
For renewal measure U with infinite mean we again use the convolution representation of Ei eγLt
to apply Lemma 1 of [E73] showing that the denominator needs to be replaced by the truncated
t s
mean 0 1 − 0 γpr (i, i) dr ds.
(3) For γ < G∞ we may directly use the proof of Proposition 3.11 of [AD09] as there no additional
structure was assumed. We repeat the simple argument for completeness. Taking Laplace trans-
form of Equation (0.3) and solving the multiplication equation in Laplace domain (note that under
i
Laplace transform the convolution turns into multiplication) we obtain with f (t) = Ei eγLt
ˆ 1 1
f (λ) =
λ 1 − γ p(λ)
ˆ
for λ > 0. As by assumption the second factor converges to the constant 1/(1 − γG∞ (i, i)) as λ
tends to zero, Karamata’s Tauberian theorem (see Theorem 1.7.6 of [BGT89]) implies the result.
Note that as f (t) is increasing, the Tauberian condition for that theorem is fulfilled.
Qualitative properties of the exponential growth rate r(κ) have been considered for the simple
random walk (see Section III of [CM94], Theorem 1.4 of [GdH06]) and in the polynomially case
(see Corollary 3.10 of [AD09]). The representation of the growth rate in the previous theorem
directly shows that the qualitative behavior (see Figure 1 for the qualitative behavior of r(γ)
plotted against the identity function) is valid for general Markov processes:
Corollary 1. Suppose (Xt ) is a time-homogeneous Markov process on S started in i. Then for
t
Li = 0 δi (Xs ) ds the following holds for γ ≥ 0:
t
(1) r(γ) ≥ 0 and r(γ) > 0 if and only if γ > 1/G∞ (i, i),
(2) the function γ → r(γ) is strictly convex for γ > 1/G∞ (i, i),
(3) r(γ) ≤ γ for all γ, and r(γ)/γ → 1, as γ → ∞.

Proof. Part (1) of Theorem 1 shows that understanding p−1 suffices to understand r(γ). This is
ˆ
not difficult due to the following observation: as p is bounded by 1, p(λ) is finite for all λ > 0,
ˆ
strictly decreasing and convex with p(0) = G∞ (i, i). Hence, p−1 (λ) = 0 if and only if λ ≥ G∞ (i, i).
ˆ ˆ

AN APPLICATION OF RENEWAL THEOREMS TO EXPONENTIAL MOMENTS OF LOCAL TIMES 5

This implies that p−1 (1/γ) = 0 precisely for λ ≤ 1/G∞ (i, i). Hence, parts (1) and (2) are proved
ˆ
as r(γ) = p−1 (1/γ).
ˆ
First note that the first part of (3) is immediate as Li ≤ t. Continuity of p and p0 (i, i) = 1 imply
t
that for > 0 there is t0 ( ) such that pt (i, i) ≥ 1 − for t ≤ t0 ( ). Hence,
∞
1
= p(r(γ)) =
ˆ e−r(γ)t pt (i, i) dt
γ 0
t0 ( )
1
≥ (1 − ) e−r(γ)t dt = (1 − ) 1 − e−r(γ)t0 ( )
.
0 r(γ)
Since r(γ) → ∞ for γ → ∞ we obtain
r(γ)
lim inf ≥ 1.
k→∞ γ
This combined with the first part of (3) proves the second part.
Let us finish with two remarks on related results for continuous space models.

In their analysis of laws of iterated logarithms for local times of symmetric L´vy processes, moment
e
generating functions of local times were considered in [MR96]. They exploited the renewal equation
(0.3) where now the transition probabilities need to be replaced by transition kernels. Solving in
Laplace domain as we did in part (3) of the proof of Theorem 1 they transformed back via inverse
Laplace transformation to estimate rather delicately the difference
i 1 ˆ−1
Ei eγLt − −1 ∞ −p−1 (1/γ)s p (i, i) ds
ˆ
ep (1/γ)t .
p (1/γ)γ 0 se
ˆ s

Their estimate is uniform in t and γ but does not establish convergence as t tends to infinity.
Applying our proofs to the renewal equation representation (see the proof of their Lemma 2.6),
we find the same results as we obtained in discrete space.

Recently, a parabolic Anderson model in R with L´vy driver was consider in [FK1] and [FK2]. As
e
their results are based on the same renewal equation (see for instance Equation (2.2) of [FK2] or
(4.15) of [FK1]) that we used, one can strengthen their bounds away from the notion of Lyapunov
exponents to strong asymptotics with the same expressions for constants and exponential rates
as in our discrete setting. This is not surprising as also for their L´vy process driven version
e
of the parabolic Anderson model the above mentioned correspondence of second moments and
exponential moments of local times of the corresponding L´vy process holds true.
e

References
[AA87] Anderson, K. K.; Athreya, K. B. ”A Renewal Theorem in the Infinite Mean Case” Annals of Probability,
15, (1987), 388-393
[BGT89] Bingham, N. H.; Goldie, C. M.; Teugels, J. L. ”Regular variation” Encyclopedia of Mathematics and its
Applications, 27, (1989), xx+494
[AD09] Aurzada, F.; D¨ring, L. ”Intermittency and Aging for the Symbiotic Branching Model” submitted
o
[CM94] Carmona, R.; Molchanov, S. ”Parabolic Anderson problem and intermittency” Mem. Amer. Math. Soc.,
108, (1994), viii+125
[DD06] Dembo, A.; Deuschel, J.D. ”Aging for Interacting Diffusion Processes” Ann. Inst. H. Poincar´ Probab.
e
Statist., 43(4), (2007), 461-480
[E73] Erickson, B. ”The strong law of large numbers when the mean is undefined ” Trans. Amer. Math. Soc., 54,
(1973), 371-381
[F71] Feller, W. ”An introduction to probability theory and its applications. Vol. II.” John Wiley & Sons, Inc.,
New York-London-Sydney, (1971), xxiv+669 pp.
[FK1] Foondun, M.; Khoshnevisan, D. ”Intermittency for nonlinear parabolic stochastic partial differential equa-
tions” Electr. Journal of Prob., 14, (2009), 548-568
[FK2] Foondun, M.; Khoshnevisan, D. ”On the global maximum of the solution to a stochastic heat equation with
compact-support initial data” Preprint
[GdH06] G¨rtner, J.; den Hollander, F. ”Intermittency in a catalytic random medium” Annals of Probability, 34
a
(6), (2006), 2219-2287.

6 ¨

[MR96] Marcus, M.; Rosen, J. ”Laws of the iterated logarithm for the local times of symmetric L´vy processes and
e
recurrent random walks” Annals of Probability, 22, (1994), 620-659

Department of Statistics, Universityt of Oxford, 1, South Parks Road, Oxford OX1 3TG, United
Kingdom
E-mail address: leif.doering@googlemail.com

Department of Statistics, Universityt of Oxford, 1, South Parks Road, Oxford OX1 3TG, United
Kingdom
E-mail address: savov@stats.ox.ac.uk

Doering Savov

More Related Content

What's hot (19)

Similar to Doering Savov (20)

Recently uploaded (20)

Doering Savov