UNBOUNDED PRODUCTS OF OPERATORS AND CONNECTIONS TO DIRAC-TYPE OPERATORS KARL GUSTAFSON1 AND MOHAMMED HICHEM MORTAD2* Abstract. Let A and B be two densely defined unbounded closeable opera- tors in a Hilbert space such that their unbounded operator products AB and BA are also densely defined. Then all four operators possess adjoints and we obtain new inclusion bounds for the operator product closures A B and AB in terms of new relations among the operator adjoints. These in turn lead to sharpened understandings for when products of unbounded self-adjoint and unbounded normal operators are self-adjoint and normal. They also clarify certain operator-product issues for Dirac operators. 1. Introduction Let A and B be two densely defined unbounded closeable operators in a Hilbert space such that their unbounded operator product domain D(AB), and often, D(BA), are also dense. These operators then possess adjoints and we obtain new results for relations among them. Our goal is to seek somewhat finer detail than we have seen in the established literature. We shall refrain inasmuch as is possible from any supplementary assumptions of boundedness, positivity and closed opera- tor ranges. In particular, this allows our results to be applicable over the continuous spectrum. See Remark 4.1. Section 2 investigates the adjoint product (AB)∗ and then establishes inclusion bounds for the operator closures A B and AB. Section 3 specializes to A and B self- adjoint and concludes with A and B normal. Although we are fully cognizant of the power and naturalness of an assumption of commutativity in such considerations, a theme there is to also examine its necessity. Section 4 elaborates key issues and gives some related literature from which further relevant bibliography may be obtained. In particular, we clarify and improve certain important results used in arXiv:1308.2427v1 [math.FA] 11 Aug 2013 investigations of Dirac operators, see [9, 39]. Thereby, this paper also provides a brief up-to-date account of these important problems. An interested reader may therefore wish to consult Section 4 while reading the earlier sections of this paper. 2. Unbounded Product Adjoints Recall that a generally unbounded operator T in a Hilbert space possesses a unique adjoint T ∗ iff D(T ) is dense. Then T is closeable with minimal closed extension T iff D(T ∗) is dense, and then (T )∗ = T ∗. When A and B and AB are densely defined, then (AB)∗ ⊃ B∗A∗, whether or not the latter adjoint operators 2010 Mathematics Subject Classification. Primary 47A05; Secondary 47B25. Key words and phrases. Unbounded operators. Self-adjoint and normal operators. Operator products. Dirac operators. * Corresponding author. 1 2 KARLGUSTAFSON1 AND MOHAMMED HICHEM MORTAD2* are densely defined. One has T = T ∗∗ for closed densely defined operators T . Moreover, if T is just densely defined and closeable, still one has T = T ∗∗. Recall adjoint operators are always closed. In the following we will always assume that our starting A and B are densely defined so that they have adjoints. A central question is when (AB)∗ = B∗A∗. Such is the case for everywhere defined bounded operators A and B in B(H). One needs only A ∈ B(H) for that equality. Another central question is when AB is closed. When A is closed and B is in B(H), then AB is closed. Also when A is invertible with A−1 in B(H), AB is closed. Some further known results may be found in the books [10, 19, 37, 42]. We digress a bit to say that similar works exist for sums of linear operators, see e.g. [16, 24, 25] and further bibliography cited therein. In this section we want to start out ”from scratch”, assuming only that A and B are densely defined closeable operators. In other words, we want to go beyond any assumptions of B(H), positivity, closed ranges, for any of our operators, inasmuch as that may be possible. Of course, we will need to make other, lesser, assumptions, as we go along. Early on, following results of J. von Neumann [33], M. A. Naimark [31] (see also [4] and [40]) showed that even a densely defined closed symmetric operator T may have domain D(T 2) not dense, even D(T 2) = {0}. Therefore we shall normally assume that D(AB) is dense, so that we may speak of (AB)∗. That D(AB) be ∞ dense is often apparent in applications, e.g. where D(AB) contains the smooth C0 functions. Let A and B be densely defined closeable operators such that AB is also densely defined. Then (AB)∗ ⊃ B∗A∗. Then the domain D((AB)∗) is also dense iff AB is closeable, and given that, then ((AB)∗)∗ exists and equals AB which has dense domain D(AB) ⊃ D(AB). Thus (AB)∗∗ is a closed densely defined operator and if D(B∗A∗) is also dense we arrive at the two inclusion relations ∗ ∗ ∗ ∗∗ (B A ) ⊃ (AB) = AB ⊃ AB and ∗ ∗ ∗ ∗∗ ∗∗ (B A ) ⊃ A B = A B ⊃ AB. We remark that the distinction between AB and A B is important and nontrivial so we want to keep it in mind even when we are thinking of closed A and B. As all of the operators mentioned are densely defined, we may again adjoint the above relations, arriving at, respectively, ∗ ∗ ∗∗∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ (AB) = (AB) = (AB) ⊃ (B A ) ⊃ B A = (B) (A) and ∗ ∗ ∗∗ ∗∗ ∗ ∗ ∗ ∗ ∗ (AB) ⊃ (A B) = (A B ) ⊃ B A = (B) (A) . Doing this again serves as both check and possible refinement of these considera- tions, from which we have, respectively, ∗∗ ∗ ∗ ∗∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ AB = (AB) ⊂ (B A ) = (B A ) = (B∗A∗) = (B A ) , and ∗∗ ∗∗ ∗ ∗ ∗ AB ⊂ (A B) = (A B) = A B ⊂ (B A ) . Let us summarize the relations obtained to this point. UNBOUNDEDPRODUCTSOFOPERATORS 3 Lemma 1. Given densely defined closeable A and B such that D(AB) is dense, we have (AB)∗ ⊃ B∗A∗. If also D(B∗A∗) is dense, we have all of the above relations. In particular, AB and A B are both included between AB and (B∗A∗)∗. Of course one could see that AB and A B are both sandwiched between AB and (B∗A∗)∗ directly from ∗ ∗ ∗ ∗∗ ∗∗ (B A ) ⊃ A B = A B, i.e., from the general adjoint property, and from the general property that the operator closure is always minimal, hence ∗ ∗ ∗ (B A ) ⊃ AB. But we wanted also the finer details of the Lemma available to us. From the above it is clear that there are intimate relationships between product adjoints and product closures and closures of products. In particular, we may ask the consequences of when equality may hold in the above relations. For example, suppose equality holds in the fundamental adjoint operation ∗ ∗ ∗ ∗∗ ∗∗ (B A ) = A B . If A and B be closed, that means AB is closed. Moreover, then (AB)∗ = B∗A∗. Should B∗A∗ also be closed, then the fundamental adjoint relation (AB)∗ = B∗A∗ obtains. Similarly, suppose equality holds in the other above obtained general adjoint relation (B∗A∗)∗ ⊃ AB, i.e., that ∗ ∗ ∗ (B A ) = AB. Then since (AB)∗ = (AB)∗ we have (AB)∗ = B∗A∗, whether or not A and B are closed. Should B∗A∗ also be closed, then again the fundamental adjoint relation (AB)∗ = B∗A∗ holds. Let us summarize a version of these observations. Theorem 1. Let A and B be densely defined closeable operators such that D(AB) and D(B∗A∗) are both dense. Then the product adjoint equality (B∗A∗)∗ = A∗∗B∗∗ necessitates that the product A B be closed and that (A B)∗ = (B∗A∗). If moreover A and B are closed, then AB is necessarily closed. If, further, the product B∗A∗ is closed, then the fundamental adjoint relation ∗ ∗ ∗ (AB) = B A obtains. Similarly, equality in the other adjoint relation above, (B∗A∗)∗ = AB, along with closure of the product B∗A∗, also yields the sought adjoint relation (AB)∗ = B∗A∗. Conversely, necessary conditions for that fundamental adjoint relation include: B∗A∗ closed, and AB ⊃ A B. Remark. Assuming B∗A∗ closed may not be merely dropped. For if A and B are self-adjoint, and B is the bounded inverse of A, then AB = I and BA ⊂ I. Hence (AB)∗ 6= BA. An explicit example would be: Example 1. Let A be the unbounded operator defined by Af(x)=(1+ x2)f(x) for all f ∈ L2(R) on the domain D(A) = {f ∈ L2(R): x2f ∈ L2(R)}. Then it is clear that A is self-adjoint and positive on D(A). Let B be its inverse, i.e. 1 Bf(x)= f(x), ∀f ∈ L2(R). 1+ x2 We may recover the following well-known result (due to von-Neumann) 4 KARLGUSTAFSON1 AND MOHAMMED HICHEM MORTAD2* Corollary 1. If A is a densely defined closed operator, then AA∗ and A∗A are self-adjoint. From the above analysis, one is naturally led to wonder: could B∗A∗ closed and densely defined provide a sufficient condition for the sought (AB)∗ = B∗A∗ fundamental adjoint product relation? After all, if A and B are also closed, then if (B∗A∗)∗ = A∗∗B∗∗ = AB, and adjointing again yields the sought (AB)∗ = B∗A∗. The answer is: yes, usually. This follows from the following fundamental result obtained by one of us long ago [12]. Theorem 2. Let T and S be densely defined linear operators in a Hilbert space. If S is closed and range R(S) has finite codimension, then (TS)∗ = S∗T ∗. Corollary 2. Let A and B be closed densely defined operators with D(AB) and D(B∗A∗) dense. Suppose also that B∗A∗ is closed and densely defined and R(A∗) is closed.