In the mathematical field of real analysis, the monotone convergence theorem is any of a number of related theorems proving the convergence of monotonic sequences (sequences that are increasing or decreasing) that are also bounded. Informally, the theorems state that if a sequence is increasing and bounded above by a supremum, then the sequence will converge to the supremum; in the same way, if a sequence is decreasing and is bounded below by an infimum, it will converge to the infimum.
Contents
Lemma 1
If a sequence of real numbers is increasing and bounded above, then its supremum is the limit.
Proof
prove that if an increasing sequenceSince
Lemma 2
If a sequence of real numbers is decreasing and bounded below, then its infimum is the limit.
Proof
The proof is similar to the proof for the case when the sequence is increasing and bounded above.
Theorem
If
Proof
Theorem
If for all natural numbers j and k, aj,k is a non-negative real number and aj+1,k ≤ aj,k, then
The theorem states that if you have an infinite matrix of non-negative real numbers such that
- the columns are weakly increasing and bounded, and
- for each row, the series whose terms are given by this row has a convergent sum,
then the limit of the sums of the rows is equal to the sum of the series whose term k is given by the limit of column k (which is also its supremum). The series has a convergent sum if and only if the (weakly increasing) sequence of row sums is bounded and therefore convergent.
As an example, consider the infinite series of rows
where n approaches infinity (the limit of this series is e). Here the matrix entry in row n and column k is
the columns (fixed k) are indeed weakly increasing with n and bounded (by 1/k!), while the rows only have finitely many nonzero terms, so condition 2 is satisfied; the theorem now says that you can compute the limit of the row sums
Lebesgue's monotone convergence theorem
This theorem generalizes the previous one, and is probably the most important monotone convergence theorem. It is also known as Beppo Levi's theorem.
Theorem
Let (X, Σ, μ) be a measure space. Let
Next, set the pointwise limit of the sequence
Then f is Σ–measurable and
Remark. If the sequence
provided that f is Σ-measurable.
Proof
We will first show that f is Σ–measurable. To do this, it is sufficient to show that the inverse image of an interval [0, t] under f is an element of the sigma algebra Σ on X, because (closed) intervals generate the Borel sigma algebra on the reals. Let I = [0, t] be such a subinterval of [0, ∞]. Let
Since I is a closed interval and
Thus,
Note that each set in the countable intersection is an element of Σ because it is the inverse image of a Borel subset under a Σ-measurable function
Now we will prove the rest of the monotone convergence theorem. The fact that f is Σ-measurable implies that the expression
We will start by showing that
By the definition of the Lebesgue integral,
where SF is the set of Σ-measurable simple functions on X. Since
Hence, since the supremum of a subset cannot be larger than that of the whole set, we have that:
and the limit on the right exists, since the sequence is monotonic.
We now prove the inequality in the other direction (which also follows from Fatou's lemma), that is we seek to show that
It follows from the definition of integral, that there is a non-decreasing sequence (gk) of non-negative simple functions such that gk ≤ f and such that
It suffices to prove that for each
because if this is true for each k, then the limit of the left-hand side will also be less than or equal to the right-hand side.
We will show that if gk is a simple function and
for every x, then
Since the integral is linear, we may break up the function
To prove this result, fix ε > 0 and define the sequence of measurable sets
By monotonicity of the integral, it follows that for any
By the assumption that
Thus, we have that
Using the monotonicity property of measures, we can continue the above equalities as follows:
Taking k → ∞, and using the fact that this is true for any positive ε, the result follows.