A sequence of stochastic processes, , is said to converge to a process X under the semimartingale topology, as n goes to infinity, if the following conditions are met. First, should tend to in probability. Also, for every sequence of elementary predictable processes with ,
in probability for all times t. For short, this will be denoted by .
The semimartingale topology is particularly well suited to the class of semimartingales, and to stochastic integration. Previously, it was shown that the cadlag and adapted processes are complete under semimartingale convergence. In this post, it will be shown that the set of semimartingales is also complete. That is, if a sequence of semimartingales converge to a limit X under the semimartingale topology, then X is also a semimartingale.
Theorem 1 The space of semimartingales is complete under the semimartingale topology.
The same is true of the space of stochastic integrals defined with respect to any given semimartingale. In fact, for a semimartingale X, the set of all processes which can be expressed as a stochastic integral can be characterized as follows; it is precisely the closure, under the semimartingale topology, of the set of elementary integrals of X. This result was originally due to Memin, using a rather different proof to the one given here. The method used in this post only relies on the elementary properties of stochastic integrals, such as the dominated convergence theorem.
Theorem 2 Let X be a semimartingale. Then, a process Y is of the form for some if and only if there is a sequence of bounded elementary processes with .
Writing S for the set of processes of the form for bounded elementary , and for its closure under the semimartingale topology, the statement of the theorem is equivalent to

(1) 
Recall that the semimartingale topology is generated by a translation invariant metric satisfying the following basic properties.
 .
 for all real numbers .
To verify that such a semimetric generates a vector topology, so that addition of processes and multiplication by scalars is continuous, the only other required property is that for all sequences of real numbers . In fact, this property precisely picks out the semimartingales.
Theorem 3 A cadlag adapted process X is a semimartingale if and only if for all sequences of real numbers .
Proof: The condition that is equivalent to the property that, for all sequences of elementary processes with , the integrals tend to zero in probability. By the sequential characterization of boundedness, this is equivalent to being bounded in probability for each t. As previously shown, this property characterizes semimartingales.
It follows that the semimartingales form a topological vector space.
Corollary 4 Semimartingale convergence is a vector topology on the space of semimartingales.
We now move on to the proofs of the completeness theorems. The starting point is to show that every bounded predictable integrand can be approximated by bounded elementary integrands in the following sense.
Lemma 5 Let X be a semimartingale and be a predictable process with for some constant K. Then, there is a sequence of elementary processes with and such that .
Proof: Let S be the set of processes of the form for elementary . The lemma states that is in the semimartingale closure for all predictable .
Let A be the set of predictable processes such that . This contains the elementary processes. Furthermore, if is a sequence converging to a limit , then dominated convergence gives
So, . Then, the monotone class theorem says that all predictable processes are in A.
Next, stochastic integration of a bounded predictable process is a Lipschitz continuous map on the semimartingales.
Lemma 6 Let be a constant and for cadlag X and elementary . Then .
If, furthermore, X is a semimartingale then this holds for all predictable .
Proof: Recall that where, for each t, is defined to be the supremum of
over all elementary with . In particular, if for an elementary process then is elementary and bounded by 1, giving
Taking the supremum over all such gives and, therefore, .
Now suppose that X is a semimartingale and is predictable. By Lemma 5 there exist elementary such that converge to Y under the semimartingale topology. So,
The proof of Theorem 1 for the completeness of the space of semimartingales is as follows.
Theorem 7 Let be a sequence of semimartingales and . Then, X is a semimartingale.
Proof: According to the definition of semimartingales used in these notes, it needs to be shown that there is a welldefined stochastic integral with respect to X for bounded integrands which agrees with the explicit expression for elementary integrands and satisfies bounded convergence in probability.
Using the semimartingale topology, Lemma 6 says that, for any bounded elementary , the map is continuous on the space of cadlag processes Y. So,

(2) 
If is a bounded predictable process then, again by Lemma 6, is continuous on the space of semimartingales. In particular, is Cauchy in the semimartingale topology and, by completeness for cadlag processes, converges to a limit. Then, (2) can be used to extend the integral to all bounded predictable processes.
Now suppose that is a sequence of predictable processes tending to a limit , for constant . By dominated convergence, as m goes to infinity, for each n. Finally, by Lemma 6, convergence in (2) holds uniformly over all so it follows that as required.
For the remainder of the post, let us turn our attention towards the proof of Theorem 2. If S is the set of integrals for bounded elementary , equality (1) needs to be established. We start by proving the inclusion

(3) 
which is established by the following lemma.
Proof: This is stated by Lemma 5 in the case where is bounded. Then, for general Xintegrable , dominated convergence gives
Proving the reverse inclusion to (3) is more difficult. By definition, for any , there will be bounded elementary such that . Then, the idea is to attempt to write Y as for . This sum need not converge, but we will show that it is possible to pass to a subsequence such that convergence holds absolutely outside of a negligible set.
For convenience, the notation will be used, for semimartingales X and bounded predictable . If are bounded predictable processes, setting , Lemma 6 gives the following inequality

(4) 
The following will be used to show that certain sums of predictable processes converge absolutely to an Xintegrable process.
Lemma 9 Let X be a semimartingale and be a sequence of bounded predictable processes with . Setting then, for each t,
(5)
is bounded in probability. Furthermore, if is a sequence of bounded predictable processes with then .
Proof: Setting then for any bounded predictable process ,
So, for constant , dominated convergence gives
The inequality here is just the triangle inequality combined with (4). It follows that if are real numbers and are bounded predictable processes such that either or then,
Here, the terms inside the summation are bounded by and, as this has finite sum, the limit can be commuted with the sum over n. Also, are uniformly bounded and tend to zero as . So dominated convergence is used to deduce that tends to zero as m goes to infinity. It follows that as m goes to infinity. In particular, taking and gives . Similarly, taking gives in probability for each and, by the sequential characterization of boundedness, the set in (5) is bounded in probability as required.
Next, the sum in the above lemma is finite outside of a negligible set.
Lemma 10 Let X and be as in Lemma 9. Then, the set is predictable and
for all bounded predictable .
Proof: The fact that A is predictable follows from the condition that are predictable. Now choose any bounded predictable , any positive time t, and set . If is a sequence of real numbers tending to infinity then, as on A, the processes are all bounded by . Then, Lemma 9 says that the sequence is bounded in probability. However, by construction, as whenever . So, U=0 almost surely, as required.
Finally, the reverse inclusion to (3) can be shown, completing the proof of Theorem 2.
Lemma 11 Let X be a semimartingale, S be the set of integrals for bounded elementary , and be the semimartingale closure of S. Then,
Proof: By definition, for any there is a sequence of bounded elementary such that . So, as m,n go to infinity. Equivalently, . Passing to a subsequence if necessary, we may suppose that .
Then, set and . The sum
is finite. So, setting , Lemma 9 shows that all sequences of bounded predictable processes with satisfy . Therefore, is Xintegrable. Furthermore, is absolutely convergent outside of A. Assuming, without loss of generality, that is identically zero, define the following limit
As are bounded by the Xintegrable process , will be Xintegrable and, by dominated convergence
Finally, Lemma 10 gives so,
as required.
Notes
Completeness of the set of stochastic integrals with respect to a semimartingale was originally stated by Memin, in the paper Espaces de semi martingales et changement de probabilité. This uses quite different techniques to those employed in this post. In Memin’s approach, stochastic integration was defined with respect to decompositions into local martingale and finite variation terms. Completeness of stochastic integrals for the local martingale and finite variation components can be inferred from completeness of spaces. Then, changes of measure are used to prove completeness for arbitrary semimartingales.
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