Reverse Fatou Lemma

In mathematics, Fatou's lemma establishes an inequality relating the Lebesgue integral of the limit inferior of a sequence of functions to the limit inferior of integrals of these functions. The lemma is named after Pierre Fatou.

Fatou's lemma can be used to prove the Fatou–Lebesgue theorem and Lebesgue's dominated convergence theorem.

Standard statement

In what follows, $\operatorname_$ denotes the $\sigma$-algebra of Borel sets on ,+\infty/math>.

Fatou's lemma remains true if its assumptions hold $\mu$-almost everywhere. In other words, it is enough that there is a null set $N$ such that the values $\$ are non-negative for every $.$ To see this, note that the integrals appearing in Fatou's lemma are unchanged if we change each function on $N$.

Proof

Fatou's lemma does ''not'' require the monotone convergence theorem, but the latter can be used to provide a quick proof. A proof directly from the definitions of integrals is given further below.

In each case, the proof begins by analyzing the properties of $\textstyle g_n(x)=\inf_f_k(x)$. These satisfy:
# the sequence $\_n$ is pointwise non-decreasing at any and
# $g_n\leq f_n$, $\forall n \in \N$.
Since $f(x) =\liminf_ f_n(x) = \lim_ \inf_ f_k(x)=\lim_$, we immediately see that is measurable.

Via the Monotone Convergence Theorem

Moreover,
:$\int_X f\,d\mu=\int_X\lim_g_n\,d\mu$
By the Monotone Convergence Theorem and property (1), the limit and integral may be interchanged:
:$\begin \int_X f\,d\mu&=\lim_\int_X g_n\,d\mu\\ &=\liminf_\int_X g_n\,d\mu\\ &\leq \liminf_\int_X f_n\,d\mu, \end$
where the last step used property (2).

From "first principles"

To demonstrate that the monotone convergence theorem is not "hidden", the proof below does not use any properties of Lebesgue integral except those established here.

Denote by $\operatorname(f)$ the set of simple $(\mathcal, \operatorname_)$-measurable functions s:X\to Borel σ-algebra and the Lebesgue measure.
* Example for a probability space: Let $S=[0,1]$ denote the unit interval. For every natural number $n$ define
::$f_n(x)=\beginn&\textx\in (0,1/n),\\ 0&\text \end$
* Example with uniform convergence: Let $S$ denote the set of all real numbers. Define
::
f_n(x)=\begin\frac1n&\textx\in ,n\\
0&\text
\end

These sequences $(f_n)_$ converge on $S$ pointwise (respectively uniformly) to the zero function (with zero integral), but every $f_n$ has integral one.

The role of non-negativity

A suitable assumption concerning the negative parts of the sequence ''f''₁, ''f''₂, . . . of functions is necessary for Fatou's lemma, as the following example shows. Let ''S'' denote the half line ,∞)_with_the_Borel_σ-algebra_and_the_Lebesgue_measure._For_every_natural_number_''n''_define
:$f_n(x)=\begin-\frac1n&\textx\in_[n,2n$\\
0&\text
\end
This_sequence_converges_uniformly_on_''S''_to_the_zero_function_and_the_limit,_0,_is_reached_in_a_finite_number_of_steps:_for_every_''x'' ≥ 0,_if_,_then_''f_n''(''x'') = 0.__However,_every_function_''f_n''_has_integral_−1.__Contrary_to_Fatou's_lemma,_this_value_is_strictly_less_than_the_integral_of_the_limit_(0).__

As_discussed_in__below,_the_problem_is_that_there_is_no_uniform_integrable_bound_on_the_sequence_from_below,_while_0_is_the_uniform_bound_from_above.

_Reverse_Fatou_lemma

Let_''f''₁,_''f''₂,_. . ._be_a_sequence_of_[ ,∞)_with_the_Borel_σ-algebra_and_the_Lebesgue_measure._For_every_natural_number_''n''_define
:$f_n(x)=\begin-\frac1n&\textx\in_[n,2n$\\
0&\text
\end
This_sequence_converges_uniformly_on_''S''_to_the_zero_function_and_the_limit,_0,_is_reached_in_a_finite_number_of_steps:_for_every_''x'' ≥ 0,_if_,_then_''f_n''(''x'') = 0.__However,_every_function_''f_n''_has_integral_−1.__Contrary_to_Fatou's_lemma,_this_value_is_strictly_less_than_the_integral_of_the_limit_(0).__

As_discussed_in__below,_the_problem_is_that_there_is_no_uniform_integrable_bound_on_the_sequence_from_below,_while_0_is_the_uniform_bound_from_above.

_Reverse_Fatou_lemma

Let_''f''₁,_''f''₂,_. . ._be_a_sequence_of_extended_real_number_line">extended_real-valued_measurable_functions_defined_on_a_measure_space_(''S'',''Σ'',''μ'')._If_there_exists_a_non-negative_integrable_function_''g''_on_''S''_such_that_''f''_''n'' ≤ ''g''_for_all_''n'',_then
:$\limsup_\int_S_f_n\,d\mu\leq\int_S\limsup_f_n\,d\mu.$

Note:_Here_''g integrable''_means_that_''g''_is_measurable_and_that_$\textstyle\int_S_g\,d\mu<\infty$.

_Sketch_of_proof

We_apply_linearity_of_Lebesgue_integral_and_Fatou's_lemma_to_the_sequence_$g_-_f_n.$__Since_$\textstyle\int_Sg\,d\mu_<_+\infty,$_this_sequence_is_defined_$\mu$-almost_everywhere_and_non-negative.

_Extensions_and_variations_of_Fatou's_lemma

_Integrable_lower_bound

Let_''f''₁,_''f''₂,_. . ._be_a_sequence_of_extended_real-valued_measurable_functions_defined_on_a_measure_space_(''S'',''Σ'',''μ'')._If_there_exists_an_integrable_function_''g''_on_''S''_such_that_''f''_''n'' ≥ −''g''_for_all_''n'',_then
:$\int_S_\liminf__f_n\,d\mu _\le_\liminf__\int_S_f_n\,d\mu.$

_Proof

Apply_Fatou's_lemma_to_the_non-negative_sequence_given_by_''f''_''n'' + ''g''.

_Pointwise_convergence

If_in_the_previous_setting_the_sequence_''f''₁,_''f''₂,_. . ._Pointwise_convergence.html" ;"title="xtended_real_number_line.html" ;"title=",2n.html" ;"title=",∞) with the Borel σ-algebra and the Lebesgue measure. For every natural number ''n'' define
:
f_n(x)=\begin-\frac1n&\textx\in [n,2n">,∞) with the Borel σ-algebra and the Lebesgue measure. For every natural number ''n'' define
:$f_n(x)=\begin-\frac1n&\textx\in [n,2n\\ 0&\text \end$
This sequence converges uniformly on ''S'' to the zero function and the limit, 0, is reached in a finite number of steps: for every ''x'' ≥ 0, if , then ''f_n''(''x'') = 0. However, every function ''f_n'' has integral −1. Contrary to Fatou's lemma, this value is strictly less than the integral of the limit (0).

As discussed in below, the problem is that there is no uniform integrable bound on the sequence from below, while 0 is the uniform bound from above.

Reverse Fatou lemma

Let ''f''₁, ''f''₂, . . . be a sequence of extended real number line">extended real-valued measurable functions defined on a measure space (''S'',''Σ'',''μ''). If there exists a non-negative integrable function ''g'' on ''S'' such that ''f''_''n'' ≤ ''g'' for all ''n'', then
:$\limsup_\int_S f_n\,d\mu\leq\int_S\limsup_f_n\,d\mu.$

Note: Here ''g integrable'' means that ''g'' is measurable and that $\textstyle\int_S g\,d\mu<\infty$.

Sketch of proof

We apply linearity of Lebesgue integral and Fatou's lemma to the sequence $g - f_n.$ Since $\textstyle\int_Sg\,d\mu < +\infty,$ this sequence is defined $\mu$-almost everywhere and non-negative.

Extensions and variations of Fatou's lemma

Integrable lower bound

Let ''f''₁, ''f''₂, . . . be a sequence of extended real-valued measurable functions defined on a measure space (''S'',''Σ'',''μ''). If there exists an integrable function ''g'' on ''S'' such that ''f''_''n'' ≥ −''g'' for all ''n'', then
:$\int_S \liminf_ f_n\,d\mu \le \liminf_ \int_S f_n\,d\mu.$

Proof

Apply Fatou's lemma to the non-negative sequence given by ''f''_''n'' + ''g''.

Pointwise convergence

If in the previous setting the sequence ''f''₁, ''f''₂, . . . Pointwise convergence">converges pointwise to a function ''f'' ''μ''-almost everywhere on ''S'', then
:$\int_S f\,d\mu \le \liminf_ \int_S f_n\,d\mu\,.$

Proof

Note that ''f'' has to agree with the limit inferior of the functions ''f''_''n'' almost everywhere, and that the values of the integrand on a set of measure zero have no influence on the value of the integral.

Convergence in measure

The last assertion also holds, if the sequence ''f''₁, ''f''₂, . . . converges in measure to a function ''f''.

Proof

There exists a subsequence such that
:$\lim_ \int_S f_\,d\mu=\liminf_ \int_S f_n\,d\mu.$
Since this subsequence also converges in measure to ''f'', there exists a further subsequence, which converges pointwise to ''f'' almost everywhere, hence the previous variation of Fatou's lemma is applicable to this subsubsequence.

Fatou's Lemma with Varying Measures

In all of the above statements of Fatou's Lemma, the integration was carried out with respect to a single fixed measure μ. Suppose that μ_n is a sequence of measures on the measurable space (''S'',''Σ'') such that (see Convergence of measures)
:$\mu_n(E)\to \mu(E),~\forall E\in \mathcal.$
Then, with ''f_n'' non-negative integrable functions and ''f'' being their pointwise limit inferior, we have
:$\int_S f\,d\mu \leq \liminf_ \int_S f_n\, d\mu_n.$

:

Fatou's lemma for conditional expectations

In probability theory, by a change of notation, the above versions of Fatou's lemma are applicable to sequences of random variables ''X''₁, ''X''₂, . . . defined on a probability space $\scriptstyle(\Omega,\,\mathcal F,\,\mathbb P)$; the integrals turn into expectations. In addition, there is also a version for conditional expectations.

Standard version

Let ''X''₁, ''X''₂, . . . be a sequence of non-negative random variables on a probability space $\scriptstyle(\Omega,\mathcal F,\mathbb P)$ and let
$\scriptstyle \mathcal G\,\subset\,\mathcal F$ be a sub-σ-algebra. Then
:\mathbb\Bigl \,\mathcal G\Bigrle\liminf_\,\mathbb \mathcal G/math>   almost surely.

Note: Conditional expectation for non-negative random variables is always well defined, finite expectation is not needed.

Proof

Besides a change of notation, the proof is very similar to the one for the standard version of Fatou's lemma above, however the monotone convergence theorem for conditional expectations has to be applied.

Let ''X'' denote the limit inferior of the ''X''_''n''. For every natural number ''k'' define pointwise the random variable
:$Y_k=\inf_X_n.$
Then the sequence ''Y''₁, ''Y''₂, . . . is increasing and converges pointwise to ''X''.
For ''k'' ≤ ''n'', we have ''Y''_''k'' ≤ ''X''_''n'', so that
:\mathbb \mathcal Gle\mathbb \mathcal G/math>   almost surely
by the monotonicity of conditional expectation, hence
:\mathbb \mathcal Gle\inf_\mathbb \mathcal G/math>   almost surely,
because the countable union of the exceptional sets of probability zero is again a null set.
Using the definition of ''X'', its representation as pointwise limit of the ''Y''_''k'', the monotone convergence theorem for conditional expectations, the last inequality, and the definition of the limit inferior, it follows that almost surely
:
\begin
\mathbb\Bigl \,\mathcal G\Bigr&=\mathbb \mathcal G=\mathbb\Bigl \,\mathcal G\Bigr=\lim_\mathbb \mathcal G\
&\le\lim_ \inf_\mathbb \mathcal G=\liminf_\,\mathbb \mathcal G
\end

Extension to uniformly integrable negative parts

Let ''X''₁, ''X''₂, . . . be a sequence of random variables on a probability space $\scriptstyle(\Omega,\mathcal F,\mathbb P)$ and let
$\scriptstyle \mathcal G\,\subset\,\mathcal F$ be a sub-σ-algebra. If the negative parts

:$X_n^-:=\max\,\qquad n\in,$

are uniformly integrable with respect to the conditional expectation, in the sense that, for ''ε'' > 0 there exists a ''c'' > 0 such that

:\mathbb\bigl \,\mathcal G\bigr\varepsilon,
\qquad\textn\in\mathbb,\,\text,
then

:\mathbb\Bigl \,\mathcal G\Bigrle\liminf_\,\mathbb \mathcal G/math>   almost surely.

Note: On the set where

:$X:=\liminf_X_n$

satisfies

:\mathbb \,\mathcal G\infty,

the left-hand side of the inequality is considered to be plus infinity. The conditional expectation of the limit inferior might not be well defined on this set, because the conditional expectation of the negative part might also be plus infinity.

Proof

Let ''ε'' > 0. Due to uniform integrability with respect to the conditional expectation, there exists a ''c'' > 0 such that

:\mathbb\bigl \,\mathcal G\bigr\varepsilon
\qquad\textn\in\mathbb,\,\text.

Since

:$X+c\le\liminf_(X_n+c)^+,$

where ''x''⁺ := max denotes the positive part of a real ''x'', monotonicity of conditional expectation (or the above convention) and the standard version of Fatou's lemma for conditional expectations imply

:\mathbb \,\mathcal Gc
\le\mathbb\Bigl \,\mathcal G\Bigr\le\liminf_\mathbb \,\mathcal G/math>   almost surely.

Since

:$(X_n+c)^+=(X_n+c)+(X_n+c)^-\le X_n+c+X_n^-1_,$

we have

:\mathbb \,\mathcal G\le\mathbb \,\mathcal Gc+\varepsilon   almost surely,

hence

:\mathbb \,\mathcal Gle
\liminf_\mathbb \,\mathcal G\varepsilon   almost surely.

This implies the assertion.

References

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