# Weak Poincaré Inequalities in the Absence of Spectral Gaps

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## Abstract

For generators of Markov semigroups which lack a spectral gap, it is shown how bounds on the density of states near zero lead to a so-called weak Poincaré inequality (WPI), originally introduced by Liggett (Ann Probab 19(3):935–959, 1991). Applications to general classes of constant coefficient pseudodifferential operators are studied. Particular examples are the heat semigroup and the semigroup generated by the fractional Laplacian in the whole space, where the optimal decay rates are recovered. Moreover, the classical Nash inequality appears as a special case of the WPI for the heat semigroup.

## Mathematics Subject Classification

39B62 37A30 35J05 47D07## 1 Introduction and Statement of Results

In this note, we study how the well-known equivalence between spectral gaps, Poincaré inequalities and exponential rates of decay to equilibrium extends to systems which lack a spectral gap but have a bounded density of states near 0. Our main result relies solely on our ability to “differentiate” the resolution of the identity of a given operator. It is thus quite general and covers important examples such as Markov semigroups.

*M*be a manifold with Borel measure \(\mathrm {d}\mu \), \(\mathcal {H}=L^2(M,\mathrm {d}\mu ;\mathbb {R})\) equipped with scalar product \((\cdot ,\cdot )_\mathcal {H}\). We assume that \(H:D(H)\subset \mathcal {H}\rightarrow \mathcal {H}\) is a self-adjoint, nonnegative operator, so that \(-H\) is the infinitesimal generator of a Markov semigroup \((P_t)_{t\ge 0}\), whose invariant measure is \(\mathrm {d}\mu \), i.e., for every

*u*that is bounded and nonnegative \(\int _M P_tu\,\mathrm {d}\mu =\int _M u\,\mathrm {d}\mu \) for any \(t\ge 0.\) Let \(\{E(\lambda )\}_{\lambda \ge 0}\) be the resolution of the identity of

*H*, and let the associated Dirichlet form be

*H*has continuous spectrum in a neighborhood of 0. (And 0 itself is possibly an eigenvalue.) We show that an appropriate estimate of the density of the spectrum near 0 leads to a weaker version of the Poincaré inequality (also known as a weak Poincaré inequality, defined in Definition 1.3). This, in turn, leads to an algebraic decay rate for the associated semigroup.

*H*. In the case where the kernel only consists of constant functions and \(\mu \) is a probability measure, this definition coincides with the standard definition, see [3, §4.2.1]. We discuss the significance of the resolution of the identity of

*H*(and in particular the projection onto its kernel) and its relationship with functional inequalities and decay rates in Sect. 2.3.

We can now recall the classical Poincaré inequality (again, see [3, §4.2.1]):

### Definition 1.1

*Poincaré Inequality*). We say that

*H*satisfies a

*Poincaré inequality*if there exists \(C>0\) such that

*C*does not depend on

*u*.

### Remark 1.2

The topology of \(D(\mathcal {E})\) is the graph norm topology generated by \(\Vert \cdot \Vert _\mathcal {H}^2+\mathcal {E}(\cdot )\), see [3, §3.1.4].

The definition of a “weak Poincaré inequality” is somewhat ambiguous. This is addressed in further detail in Sect. 2.3. We adopt the following definition, motivated by Liggett [13, Equation (2.3)]:

### Definition 1.3

*Weak Poincaré Inequality*). Let \(\Phi :\mathcal {H}\rightarrow [0,\infty ]\) satisfy \(\Phi (u)<\infty \) on a dense subset of \(D(\mathcal {E})\). Let \(p\in (1,\infty )\). We say that

*H*satisfies a \((\Phi ,{p})\)-

*weak Poincaré inequality*(\((\Phi ,{p})\)-

*WPI*) if there exists \(C>0\) such that

*C*does not depend on

*u*and where \(1/p+1/q=1\).

### Remark 1.4

Note that (1.1) is meaningful only on a dense subset of \(D(\mathcal {E})\) where \(\Phi <+\infty \).

### 1.1 The Hilbertian Case

We start our discussion by considering the purely Hilbertian case, i.e., we consider generators with density of states that are defined on subspaces which respect the Hilbert structure of \(\mathcal {H}\), such as Sobolev spaces or weighted spaces. Our basic assumption is:

### Assumption A1

- (1)
\(\mathcal {X}\cap D(\mathcal {E})\) is dense in \(D(\mathcal {E})\) (in the topology of \(D(\mathcal {E})\)),

- (2)
for some constants \(r>0\), \(C_1>0\) and \(\alpha >-1\),

*r*) for every \(u,v\in \mathcal {X}\) and satisfies

### Remark 1.5

We refer to the bilinear form \(\frac{\mathrm {d}}{\mathrm {d}\lambda }(E(\lambda )\cdot ,\cdot )_{\mathcal {H}}\) as the density of states (DoS) of *H* at \(\lambda \). Note that if the DoS satisfies a bound as in (1.2) and \(\mathcal {X}\) has a norm compatible with (and stronger than) the norm on \(\mathcal {H}\), then it induces an operator \(\mathcal {X}\rightarrow \mathcal {X}^*\) by the Riesz representation theorem.

We can finally state our main results on how (1.2) leads to a \((\Phi ,p)\)-WPI (Theorem 1.6) and, in turn, an explicit rate of decay (Theorem 1.7). Theorem 1.6 will be further generalized in Theorem 1.9 and then again in Proposition 1.13 where a precise constant in the WPI is obtained. The decay rates presented in Theorem 1.7 apply to the Markov semigroup generated by *H*.

### Theorem 1.6

If Assumption A1 holds, then *H* satisfies a \((\Phi ,p)\)-weak Poincaré inequality with \(\Phi (u)=\Vert u\Vert _{\mathcal {X}}^2\) (and \(\Phi (u)=+\infty \) if \(u\in \mathcal {H}{\setminus }\mathcal {X}\)) and \(p=\frac{2+\alpha }{1+\alpha }\).

### Theorem 1.7

### Remark 1.8

- 1.
The choice of space \(\mathcal {X}\) is motivated by (1.3): it is beneficial to choose \(\mathcal {X}\) that is invariant under the Markov semigroup (i.e., if \(u\in \mathcal {X}\) then \(P_tu\in \mathcal {X}\) for all \(t\ge 0\)).

- 2.
Clearly, \(C_2(u)\) is subject to quadratic scaling, for example it can be \(C\Vert u\Vert _\mathcal {H}^2\) or \(C\Vert u\Vert _\mathcal {X}^2\), but the explicit form is not important.

### 1.2 A Generalized Theorem: Departing from the Hilbert Structure

Theorems 1.6 and 1.7 demonstrate how estimates on the density of states near 0 imply a weak Poincaré inequality and a rate of decay to equilibrium. However, it is not essential to restrict oneself to a subspace \(\mathcal {X}\). In fact, it is often desirable to deal with functional spaces that are not contained in \(\mathcal {H}\), as it may provide improved estimates and decay rates. In particular, this makes sense when the operator in question is the generator of a Markov semigroup, and acts on a range of spaces simultaneously. Hence, we replace Assumption A1 by a more general one.

### Assumption A2

*M*, a constant \(r>0\) and a function \(\psi _{\mathcal {X},\mathcal {Y}}\in L^1(0,r)\) that is strictly positive a.e. on (0,

*r*), such that

- (1)
\(\mathcal {X}\cap \mathcal {Y}\cap D(\mathcal {E})\) is dense in \(D(\mathcal {E})\) (in the topology of \(D(\mathcal {E})\)).

- (2)The mapping \(\lambda \mapsto \frac{\mathrm {d}}{\mathrm {d}\lambda }(E(\lambda )u,v)_{\mathcal {H}}\) is continuous on (0,
*r*) for every \(u\in \mathcal {X}\cap \mathcal {H}\) and \(v\in \mathcal {Y}\cap \mathcal {H}\) and satisfies$$\begin{aligned} \left| \frac{\mathrm {d}}{\mathrm {d}\lambda }(E(\lambda )u,v)_{\mathcal {H}}\right| \le \psi _{\mathcal {X},\mathcal {Y}}(\lambda )\Vert u\Vert _\mathcal {X}\Vert v\Vert _\mathcal {Y},\qquad \forall \lambda \in (0,r). \end{aligned}$$(1.5)

We can now state the following more general theorem.

### Theorem 1.9

- a.There exists \(K_0\in (0,1)\) such that the following functional inequality holds:where \(\Vert u\Vert _\mathcal {X}=+\infty \) if \(u\notin \mathcal {X}\) and similarly for \(\mathcal {Y}\).$$\begin{aligned}&(1-K)\Psi _{\mathcal {X},\mathcal {Y}}^{-1}\left( K\frac{{{\,\mathrm{Var}\,}}(u)}{\Vert u\Vert _\mathcal {X}\Vert u\Vert _\mathcal {Y}}\right) {{\,\mathrm{Var}\,}}(u)\le \mathcal {E}(u),\nonumber \\&\qquad \forall K\in (0,K_0),\,\forall u\in D(\mathcal {E}) \end{aligned}$$(1.6)
- b.
If \(\mathcal {X}=\mathcal {Y}\) and \(\psi _{\mathcal {X},\mathcal {Y}}(\lambda )=C_1\lambda ^\alpha \), \(\alpha >-1\), the estimate (1.6) reduces to the \((\Phi ,p)\)-WPI as in Definition 1.3 with \(\Phi (u)=\Vert u\Vert _\mathcal {X}^2\) and \(p=\frac{\alpha +2}{\alpha +1}\).

- c.
If, in addition, \(\mathcal {X}=\mathcal {Y}\subset \mathcal {H}\) then we obtain Theorem 1.6.

### Remark 1.10

The inequality (1.6) can be viewed as an implicit form of the weak Poincaré inequality. Note that setting \(K=0\) (which is excluded in the theorem) leads to the Poincaré inequality.

The power of this result is demonstrated in the following corollary, where the celebrated Nash inequality is obtained as a simple consequence. This simple derivation is discussed in Remark 4.2.

### Corollary 1.11

*u*. Furthermore, using Proposition 1.13 an explicit constant may be computed to yield \(C=\left( \frac{|\mathbb {S}^{d-1}|}{2} \right) ^{\frac{2}{2+d}}\frac{2+d}{d}\).

### Proof

The (simple) proof of this corollary is done by applying our results to the heat semigroup. More details are provided in the examples (Sect. 4), in particular see Remark 4.2. \(\square \)

### Remark 1.12

*r*), for some \(r>0\) (perhaps very small), is quite natural as we are interested in operators that lack a spectral gap. However, one can easily generalize our result even if that is not the case by defining

### 1.3 Precise Constants

Under additional mild assumptions, one can improve Theorem 1.9 by replacing the inequality (1.6) which contains an arbitrary constant *K* with an inequality that has an explicit constant. The question of how far this constant is from being sharp is the topic of ongoing research.

### Proposition 1.13

*R*), where \(R\in [r,+\infty ]\) is such that if \(\Psi _{\mathcal {X},\mathcal {Y}}(\rho ):=\int _0^\rho \psi _{\mathcal {X},\mathcal {Y}}(\lambda )\,\mathrm {d}\lambda \), \(\rho \in (0,R)\) and

*g*is non-decreasing and \(\lim _{\rho \rightarrow 0^{+}}g_{\mathcal {X},\mathcal {Y}}(\rho )=0\), \(\lim _{\rho \rightarrow R^{-}}g_{\mathcal {X},\mathcal {Y}}(\rho )=+\infty \). Then:

- a.The following functional inequality holds:where \(\Vert u\Vert _\mathcal {X}=+\infty \) if \(u\notin \mathcal {X}\) and similarly for \(\mathcal {Y}\).$$\begin{aligned}&\left( g_{\mathcal {X},\mathcal {Y}}^{-1}\left( \frac{{{\,\mathrm{Var}\,}}(u)}{\Vert u\Vert _\mathcal {X}\Vert u\Vert _\mathcal {Y}} \right) \right) ^2 \psi _{\mathcal {X},\mathcal {Y}}\left( g_{\mathcal {X},\mathcal {Y}}^{-1}\left( \frac{{{\,\mathrm{Var}\,}}(u)}{\Vert u\Vert _\mathcal {X}\Vert u\Vert _\mathcal {Y}} \right) \right) \Vert u\Vert _\mathcal {X}\Vert u\Vert _\mathcal {Y}\le \mathcal {E}(u),\nonumber \\&\quad \forall u\in D(\mathcal {E}), \end{aligned}$$(1.7)
- b.
If \(\mathcal {X}=\mathcal {Y}\), and \(\psi _{\mathcal {X},\mathcal {Y}}(\lambda )=C_1\lambda ^\alpha \), \(\alpha >-1\) then the estimate (1.7) reduces to the \((\Phi ,p)\)-WPI as in Definition 1.3 with \(\Phi (u)=\Vert u\Vert _\mathcal {X}^2\), \(p=\frac{\alpha +2}{\alpha +1}\) and \(C=C_1^{\frac{1}{2+\alpha }}\frac{2+\alpha }{1+\alpha }\).

**Organization of the paper.** Before proceeding to prove our theorems, we first discuss both the classical and the weak Poincaré inequalities, and their connection to Markov semigroups in Sect. 2. The proofs will follow in Sect. 3, and we then present various applications of these theorems in Sect. 4, where we shall also prove Corollary 1.11.

## 2 Poincaré Inequalities

In this section, we recall the famous Poincaré inequality, its connection to Markov semigroups, and we discuss its “weak” variant, the so-called weak Poincaré inequality.

### 2.1 The Classical Poincaré Inequality

*M*is a compact Riemannian manifold or a bounded domain of \(\mathbb {R}^d\), the classical \(L^2\) Poincaré inequality reads [3, §4.2.1]

*M*, and \(C_{M}>0\) is independent of

*u*.

*Motivation: the heat semigroup.*Let us illustrate why the quantities appearing in this inequality are natural. Let \(M\subset \mathbb {R}^d\) be a bounded, connected and smooth domain. Consider the heat semigroup, i.e., solutions of

*The entropy method.*A common method to obtain decay rates of this type is the so-called

*entropy method*. Given the “relative distance” \(\mathcal {V}\) (a Lyapunov functional), we find its

*production functional*\(\mathcal {E}\) by formally differentiating along the flow of the semigroup:

Returning to the heat semigroup, we notice that the classical Poincaré inequality (2.1) is *exactly* a functional inequality of the form of (2.3). Moreover, the linear connection between the variance and the Dirichlet form yields an exponential rate of decay for \({{\,\mathrm{Var}\,}}(P_t u_0)\).

### 2.2 Relationship to Markov Semigroups

*H*is a self-adjoint, nonnegative operator, and \(\mathrm {d}\mu \) its invariant measure. Then the Poincaré inequality, as already defined (Definition 1.1), is

### Theorem 2.1

- (1)
*H*satisfies a Poincaré inequality with constant*C*. - (2)
The spectrum of

*H*is contained in \(\left\{ 0\right\} \cup \left[ \frac{1}{C},\infty \right) \). - (3)For every \(u\in L^2\left( M,\mathrm {d}\mu \right) \) and every \(t\ge 0\),$$\begin{aligned} {{\,\mathrm{Var}\,}}(P_tu)\le e^{-2t/C}{{\,\mathrm{Var}\,}}(u). \end{aligned}$$

### 2.3 The Weak Poincaré Inequality (WPI)

It is natural to ask whether one can obtain a generalization of Theorem 2.1 to generators which lack a spectral gap. We note that a differential operator acting on functions defined in an unbounded domain (generically) lacks a spectral gap. Our Theorems 1.6 and 1.9 provide an answer to this question, where the Poincaré inequality is replaced by some form of a weak Poincaré inequality. In the following, we provide a brief review of the existing literature on variants of the weak Poincaré inequality.

*u*. Estimates of the same spirit are then developed in [9] for example.

The form of the weak Poincaré inequality which we consider (Definition 1.3) first appeared in [13, Equation (2.3)], where it is also shown how such a differential inequality leads to an algebraic decay rate. These ideas were then further developed in [2, 5, 16, 18, 19, 20, 21]. We also refer to [1] where the notion of a “weak spectral gap” is introduced.

Continuing upon the work of Röckner and Wang and their notion of WPI, works on connections between these inequalities and isoperimetry or concentration properties of the underlying measures have been extremely prolific in the probability community. We refer the interested reader to [4, 6, 8, 11, 12, 14]. For a recent account of the notions discussed here, and in particular the relationship between functional inequalities and Markov semigroups, we refer to the book [3].

## 3 Proofs of the Theorems

We first prove the more general Theorem 1.9 and show how Theorem 1.6 is a straightforward corollary. We then show how to obtain the decay rates in Theorem 1.7, and we conclude with the proof of Proposition 1.13. For brevity, we omit the subscripts from the functions \(\psi _{\mathcal {X},\mathcal {Y}}\), \(\Psi _{\mathcal {X},\mathcal {Y}}\) and \(g_{\mathcal {X},\mathcal {Y}}\).

### 3.1 Proof of Theorem 1.9a

*H*. Let \(u\in D(\mathcal {E})\cap \mathcal {X}\cap \mathcal {Y}\). Then:

*K*to be small.) Then we get

### 3.2 Proof of Theorem 1.9 b, c (and Theorem 1.6)

The proofs follow from the following lemma where we show how (1.6) leads to a \((\Phi ,p)\)-WPI.

### Lemma 3.1

When \(\mathcal {X}=\mathcal {Y}\) and \(\psi (\lambda )=C_1\lambda ^\alpha \), \(\alpha >-1\), the inequality (1.6) reduces to the \((\Phi ,p)\)-WPI with \(\Phi (u)=\Vert u\Vert _\mathcal {X}^2\) and \(p=\frac{\alpha +2}{\alpha +1}\). Furthermore, if \(\mathcal {X}=\mathcal {Y}\subset \mathcal {H}\), we recover Theorem 1.6.

### Proof

### 3.3 Proof of Theorem 1.7

### Remark 3.2

(*The constant*\(C_3\)). It is beneficial to provide a detailed computation of the constant \(C_3\) appearing in (1.4). The following computations are performed up to a constant *C* which does not depend on \(\alpha , M, \mathcal {H}, \mathcal {X}\) or any other fundamental quantity.

*aA*where \(a=\frac{1}{\alpha +1}\) and \(A=2C'\) with \(C'=\underbrace{(1-K)K^{\frac{1}{1+\alpha }}}_{{\tilde{K}}}\left( \frac{\alpha +1}{C_1}\right) ^{\frac{1}{\alpha +1}}\) where \(C_1\) and \(\alpha \) appear in the bound (1.2). We readily obtain

### 3.4 Proof of Proposition 1.13

*R*), so that

*h*is differentiable and we have

*g*increases from 0 to \(+\infty \), we see that the unique critical point, \(\rho =g^{-1}\left( \frac{a}{b} \right) \) is a maximum point of

*h*. Thus

## 4 Examples

*H*is a

**constant coefficient pseudodifferential operator**:

### Assumption A3

- (1)
\(P(0)=0\),

- (2)
\(C^{-1}|\xi |^{{\gamma _1}+1}\le P(\xi )\le C|\xi |^{{\gamma _2}}\), for any \(\xi \in \mathbb {R}^d\),

- (3)
\(C^{-1}|\xi |^{\gamma _1}\le |\nabla P(\xi )|\), for any \(\xi \in \mathbb {R}^d{\setminus }\{0\}\),

- (4)
\({\mathcal {H}}^{d-1}\left( \{\xi \in \mathbb {R}^d:P(\xi )=\lambda \}\right) \le C\lambda ^{\frac{d-1}{{\gamma _1}+1}}\), for any \(\lambda >0\).

*C*in all inequalities for simplicity, but one could specify different constants.)

*H*:

### Remark 4.1

*Other functional subspaces*). We focus here on solutions lying in \(L^1\). However, other natural subspaces to consider are the Hilbert subspaces \(L^{2,s}(\mathbb {R}^d)\), defined as

### 4.1 The Laplacian

### Remark 4.2

(*Nash inequality*). This functional inequality is precisely the Nash inequality. This demonstrates how our methodology gives a general framework for many known important inequalities, presented in general form in (4.3) and (4.4).

### Remark 4.3

(*The constant in the Nash inequality*). We note that the computation (4.2) can be performed with precise constants in this case. Then, using Proposition 1.13, we may extract a precise constant in (4.5). A simple computation yields the constant \(C=\left( \frac{|\mathbb {S}^{d-1}|}{2} \right) ^{\frac{2}{2+d}}\frac{2+d}{d}\). These computations are left to the reader. We note that the optimal constant in the Nash inequality has already been obtained long ago by Carlen and Loss [10]. Improving our constant is the subject of ongoing research.

**Convergence to equilibrium.**We can apply Theorems 1.9c and 1.7 with \(\alpha =\frac{d}{2}-1\) and \(\Phi (u)=\Vert u\Vert _{L^1}^2\). Using the fact that the \(L^1\) norm of solutions to the heat equation does not increase, we have \(C_2=0\), where \(C_2\) is the constant appearing in (1.3). The bound (1.4) becomes

### 4.2 The Fractional Laplacian

### Remark 4.4

There is no reason not to take values of *p* greater than 1. However, the restriction to \(p\in (0,1)\) is quite common in the literature, and the result below on time decay only applies to \(p\in (0,1)\).

**Convergence to equilibrium.**From [7] we know that \(\Vert u(t,\cdot )\Vert _{L^1}\le \Vert u_0\Vert _{L^1}\) and as such, much like the previous example, we conclude that

### 4.3 Homogeneous Elliptic Operators

*assumed*to be such that the operator satisfies Assumption A3. In this case, \(m=\gamma _1+1=\gamma _2\) and the WPI (4.4) becomes

- (1)
\(P(\xi )=\sum _{i=1}^d|\xi _i|^4\),

- (2)
\(P(\xi )=\sum _{i=1}^d|\xi _i|^2-\xi _1\xi _2\).

**Convergence to equilibrium.** In order to prove convergence to an equilibrium state, one has to know how the \(L^1\) norm behaves under the flow. The authors are not aware of results in the literature for general operators as the ones we consider here. Based on the known results for the Laplacian and the fractional Laplacian, one could ask:

### Question 4.5

Is it true that for every homogeneous elliptic operator of order *m* which satisfies Assumption A3 and which is the generator of a semigroup \((P_t)_{t\ge 0}\) there exist \(C_2=C_2(u)\ge 0\) and \(\beta \in \mathbb {R}\) such that for every \(t\ge 0\), \(\Vert P_tu\Vert ^2_{L^1}\le \Vert u\Vert ^2_{L^1}+C_2t^\beta \)?

## Notes

### Acknowledgements

The first author was support by the UK Engineering and Physical Sciences Research Council (EPSRC) Grant EP/N020154/1. The second author was supported by the Austrian Science Fund (FWF) Grant M 2104-N32. The authors thank the referees for their useful comments which helped improve the content and presentation of this work.

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