Quasi-stationary distribution

In probability a quasi-stationary distribution is a random process that admits one or several absorbing states that are reached almost surely, but is initially distributed such that it can evolve for a long time without reaching it. The most common example is the evolution of a population: the only equilibrium is when there is no one left, but if we model the number of people it is likely to remain stable for a long period of time before it eventually collapses.

Formal definition

We consider a Markov process $(Y_{t})_{t\geq 0}$ taking values in ${\mathcal {X}}$ . There is a measurable set ${\mathcal {X}}^{\mathrm {tr} }$ of absorbing states and ${\mathcal {X}}^{a}={\mathcal {X}}\setminus {\mathcal {X}}^{\operatorname {tr} }$ . We denote by $T$ the hitting time of ${\mathcal {X}}^{\operatorname {tr} }$ , also called killing time. We denote by $\{\operatorname {P} _{x}\mid x\in {\mathcal {X}}\}$ the family of distributions where $\operatorname {P} _{x}$ has original condition $Y_{0}=x\in {\mathcal {X}}$ . We assume that ${\mathcal {X}}^{\operatorname {tr} }$ is almost surely reached, i.e. $\forall x\in {\mathcal {X}},\operatorname {P} _{x}(T<\infty )=1$ .

The general definition^[1] is: a probability measure $\nu$ on ${\mathcal {X}}^{a}$ is said to be a quasi-stationary distribution (QSD) if for every measurable set $B$ contained in ${\mathcal {X}}^{a}$ ,

\forall t\geq 0,\operatorname {P} _{\nu }(Y_{t}\in B\mid T>t)=\nu (B)

where $\operatorname {P} _{\nu }=\int _{{\mathcal {X}}^{a}}\operatorname {P} _{x}\,\mathrm {d} \nu (x)$ .

In particular $\forall B\in {\mathcal {B}}({\mathcal {X}}^{a}),\forall t\geq 0,\operatorname {P} _{\nu }(Y_{t}\in B,T>t)=\nu (B)\operatorname {P} _{\nu }(T>t).$

General results

Killing time

From the assumptions above we know that the killing time is finite with probability 1. A stronger result than we can derive is that the killing time is exponentially distributed:^[1]^[2] if $\nu$ is a QSD then there exists $\theta (\nu )>0$ such that $\forall t\in \mathbf {N} ,\operatorname {P} _{\nu }(T>t)=\exp(-\theta (\nu )\times t)$ .

Moreover, for any $\vartheta <\theta (\nu )$ we get $\operatorname {E} _{\nu }(e^{\vartheta t})<\infty$ .

Existence of a quasi-stationary distribution

Most of the time the question asked is whether a QSD exists or not in a given framework. From the previous results we can derive a condition necessary to this existence.

Let $\theta _{x}^{*}:=\sup\{\theta \mid \operatorname {E} _{x}(e^{\theta T})<\infty \}$ . A necessary condition for the existence of a QSD is $\exists x\in {\mathcal {X}}^{a},\theta _{x}^{*}>0$ and we have the equality $\theta _{x}^{*}=\liminf _{t\to \infty }-{\frac {1}{t}}\log(\operatorname {P} _{x}(T>t)).$

Moreover, from the previous paragraph, if $\nu$ is a QSD then $\operatorname {E} _{\nu }\left(e^{\theta (\nu )T}\right)=\infty$ . As a consequence, if $\vartheta >0$ satisfies $\sup _{x\in {\mathcal {X}}^{a}}\{\operatorname {E} _{x}(e^{\vartheta T})\}<\infty$ then there can be no QSD $\nu$ such that $\vartheta =\theta (\nu )$ because other wise this would lead to the contradiction $\infty =\operatorname {E} _{\nu }\left(e^{\theta (\nu )T}\right)\leq \sup _{x\in {\mathcal {X}}^{a}}\{\operatorname {E} _{x}(e^{\theta (\nu )T})\}<\infty$ .

A sufficient condition for a QSD to exist is given considering the transition semigroup $(P_{t},t\geq 0)$ of the process before killing. Then, under the conditions that ${\mathcal {X}}^{a}$ is a compact Hausdorff space and that $P_{1}$ preserves the set of continuous functions, i.e. $P_{1}({\mathcal {C}}({\mathcal {X}}^{a}))\subseteq {\mathcal {C}}({\mathcal {X}}^{a})$ , there exists a QSD.

History

The works of Wright on gene frequency in 1931^[3] and of Yaglom on branching processes in 1947^[4] already included the idea of such distributions. The term quasi-stationarity applied to biological systems was then used by Bartlett in 1957,^[5] who later coined "quasi-stationary distribution".^[6]

Quasi-stationary distributions were also part of the classification of killed processes given by Vere-Jones in 1962^[7] and their definition for finite state Markov chains was done in 1965 by Darroch and Seneta.^[8]

Examples

Quasi-stationary distributions can be used to model the following processes:

Evolution of a population by the number of people: the only equilibrium is when there is no one left.
Evolution of a contagious disease in a population by the number of people ill: the only equilibrium is when the disease disappears.
Transmission of a gene: in case of several competing alleles we measure the number of people who have one and the absorbing state is when everybody has the same.
Voter model: where everyone influences a small set of neighbors and opinions propagate, we study how many people vote for a particular party and an equilibrium is reached only when the party has no voter, or the whole population voting for it.

References

1 2 Collet, Pierre; Martínez, Servet; San Martín, Jaime (2013). Quasi-Stationary Distributions. Probability and its Applications. doi:10.1007/978-3-642-33131-2. ISBN 978-3-642-33130-5.{{cite book}}: CS1 maint: date and year (link)
↑ Ferrari, Pablo A.; Martínez, Servet; Picco, Pierre (1992). "Existence of Non-Trivial Quasi-Stationary Distributions in the Birth-Death Chain". Advances in Applied Probability. 24 (4): 795–813. doi:10.2307/1427713. JSTOR 1427713. S2CID 17018407.
↑ WRIGHT, Sewall. Evolution in Mendelian populations. Genetics, 1931, vol. 16, no 2, pp. 97–159.
↑ YAGLOM, Akiva M. Certain limit theorems of the theory of branching random processes. In : Doklady Akad. Nauk SSSR (NS). 1947. p. 3.
↑ BARTLETT, Mi S. On theoretical models for competitive and predatory biological systems. Biometrika, 1957, vol. 44, no 1/2, pp. 27–42.
↑ BARTLETT, Maurice Stevenson. Stochastic population models; in ecology and epidemiology. 1960.
↑ VERE-JONES, D. (1962-01-01). "Geometric Ergodicity in Denumerable Markov Chains". The Quarterly Journal of Mathematics. 13 (1): 7–28. Bibcode:1962QJMat..13....7V. doi:10.1093/qmath/13.1.7. hdl:10338.dmlcz/102037. ISSN 0033-5606.
↑ Darroch, J. N.; Seneta, E. (1965). "On Quasi-Stationary Distributions in Absorbing Discrete-Time Finite Markov Chains". Journal of Applied Probability. 2 (1): 88–100. doi:10.2307/3211876. JSTOR 3211876. S2CID 67838782.

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[Collet_2013-1] 1 2 Collet, Pierre; Martínez, Servet; San Martín, Jaime (2013). Quasi-Stationary Distributions. Probability and its Applications. doi:10.1007/978-3-642-33131-2. ISBN 978-3-642-33130-5.{{cite book}}: CS1 maint: date and year (link)

[2] Ferrari, Pablo A.; Martínez, Servet; Picco, Pierre (1992). "Existence of Non-Trivial Quasi-Stationary Distributions in the Birth-Death Chain". Advances in Applied Probability. 24 (4): 795–813. doi:10.2307/1427713. JSTOR 1427713. S2CID 17018407.

[3] WRIGHT, Sewall. Evolution in Mendelian populations. Genetics, 1931, vol. 16, no 2, pp. 97–159.

[4] YAGLOM, Akiva M. Certain limit theorems of the theory of branching random processes. In : Doklady Akad. Nauk SSSR (NS). 1947. p. 3.

[5] BARTLETT, Mi S. On theoretical models for competitive and predatory biological systems. Biometrika, 1957, vol. 44, no 1/2, pp. 27–42.

[6] BARTLETT, Maurice Stevenson. Stochastic population models; in ecology and epidemiology. 1960.

[7] VERE-JONES, D. (1962-01-01). "Geometric Ergodicity in Denumerable Markov Chains". The Quarterly Journal of Mathematics. 13 (1): 7–28. Bibcode:1962QJMat..13....7V. doi:10.1093/qmath/13.1.7. hdl:10338.dmlcz/102037. ISSN 0033-5606.

[8] Darroch, J. N.; Seneta, E. (1965). "On Quasi-Stationary Distributions in Absorbing Discrete-Time Finite Markov Chains". Journal of Applied Probability. 2 (1): 88–100. doi:10.2307/3211876. JSTOR 3211876. S2CID 67838782.

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