# FINITE DIMENSIONAL ORDERED VECTOR SPACES WITH RIESZ INTERPOLATION AND EFFROS-SHEN S UNIMODULARITY CONJECTURE AARON TIKUISIS

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1 FINITE DIMENSIONAL ORDERED VECTOR SPACES WITH RIESZ INTERPOLATION AND EFFROS-SHEN S UNIMODULARITY CONJECTURE AARON TIKUISIS Abstract. It is shown that, for any field F R, any ordered vector space structure of F n with Riesz interpolation is given by an inductive limit of sequence with finite stages (F n, F n 0 ) (where n does not change). This relates to a conjecture of Effros and Shen, since disproven, which is given by the same statement, except with F replaced by the integers, Z. Indeed, it shows that although Effros and Shen s conjecture is false, it is true after tensoring with Q. 1. Introduction In this article we prove the following. Theorem 1.1. Let F be a subfield of the real numbers, let n be a natural number, and suppose that (V, V + ) is a ordered directed n-dimensional vector space over F with Riesz interpolation. Then there exists an inductive system (F n, F n 0) φ 2 i (F n, F n 0) of ordered vector spaces over F whose inductive limit in (V, V + ). The inductive limit may be taken either in the category of ordered abelian groups (with positivity-preserving homomorphisms as the arrows) or of ordered vector spaces over F (with positivity-preserving linear transformations as the arrows). Here, F 0 := F [0, ), so that the ordering on (F n, F n 0) is simply given by coordinatewise comparison. In [3], Effros and Shen conjectured that every ordered, directed, unperforated, rank n free abelian group (G, G + ) with Riesz interpolation can be realized as an inductive system of ordered groups (Z n, Z n 0). This was called the unimodularity conjecture, as the connecting maps would necessarily (eventually) be unimodular. This conjecture was disproven by Riedel in [8]. Theorem 1.1 shows that, nonetheless, upon tensoring with the rational numbers (or any other field contained in R), the conjecture is true. As a consequence, Corollary 5.1 says that if 2010 Mathematics Subject Classification. 46A40, 06B75, 06F20. Key words and phrases. Dimension groups, Riesz interpolation, ordered vector spaces, unimodularity conjecture, simplicial groups, lattice-ordered groups. 1

2 2 AARON TIKUISIS (G, G + ) is an ordered n-dimensional Q-vector space with Riesz interpolation then it is an inductive limit of (Z n, Z n 0) (where the maps are, of course, not unimodular). In [6], David Handelman shows that every vector space with Riesz interpolation can be realized as an inductive limit of ordered vector spaces (F n, F n 0), though of course the number n isn t assumed to be constant among the finite stages. The focus of [6] is on the infinitedimensional case, and indeed, an interesting example is given of countable dimensional ordered vector space that can t be expressed as an inductive limit of a sequence of ordered vector spaces (F n, F n 0). Combined with this article, this gives a dichotomy between the behaviour of infinite- versus finite-dimensional ordered vector spaces with Riesz interpolation. Acknowledgements The author was supported by the Sonderforschungsbereich Preliminaries We shall say a little here about the theory of ordered vector spaces with Riesz interpolation. Although the focus in on vector spaces, much of the interesting theory holds in the more general setting of ordered abelian groups (particularly when the group is unperforated, as ordered vector spaces are automatically). An excellent account of this theory can be found in the book [4] by Ken Goodearl. Definition 2.1. An ordered vector space consists of a vector space V together with a subset V + V called the positive cone, giving an ordering compatible with the vector space structure; that is to say: (OV1) V + ( V + ) = 0 (V + gives an order, not just a preorder); (OV2) V + + V + V + ; and (OV3) λv + V + for all λ F 0. The ordering on V is of course given by x y if y x V +. The ordered vector space (V, V + ) is directed if for all x, y V, there exists z V such that x y z. The ordered vector space (V, V + ) has Riesz interpolation if for any a 1, a 2, c 1, c 2 V such that there exists b V such that a 1 a 2 c 1 c 2, a 1 a 2 b c 1 c 2.

3 ORDERED VECTOR SPACES WITH INTERPOLATION 3 Note that (V, V + ) being directed is an extremely natural condition, as it is equivalent to saying that V + V + = V. Riesz interpolation for an ordered vector space (V, V + ) is equivalent to Riesz decomposition, which says that for any x 1, x 2, y V +, if y x 1 + x 2 then there exist y 1, y 2 V + such that y = y 1 + y 2 and y i x i for i = 1, 2 [1, Section 23]. The category of ordered vector spaces (over a fixed field F) has as arrows linear transformations which are positivity-preserving, meaning that they map the positive cone of the domain into the positive cone of the codomain. This category admits inductive limits, and for an inductive system ((V α, V + α ) α A, (φ β α) α β ), the inductive limit is given concretely as (V, V + ) where V is the inductive limit of ((V α ) α A, (φ β α) α β ) in the category of vector spaces, and if φ α : V α V denotes the canonical map then V + = α A φ α (V α ). If (V α, V α + ) has Riesz interpolation for every α then so does the inductive limit (V, V + ). Theorem 1 of [6] states that every ordered F-vector space with Riesz interpolation can be realized as an inductive limit of a net of ordered vector spaces of the form (F n, F n 0). The proof uses the techniques of [2], where it was shown that every ordered directed unperforated abelian group with Riesz interpolation is an inductive limit of a net of ordered groups of the form (Z, Z 0 ). In the case that F = Q, [6, Theorem 1] follows from [2] and the theory of ordered group tensor product found in [5]. Certainly, if (V, V + ) is an ordered directed Q-vector space with Riesz interpolation then it can be written as an inductive limit of G α = (Z nα, Z nα 0 ), and then we have (V, V + ) = (Q, Q + ) Z (V, V + ) = lim(q, Q 0 ) Z (Z nα, Z nα 0 ) = lim(q α, Q nα 0 ). But in the case of other fields, we no longer have (V, V + ) = (V, V + ) Z (F, F 0 ) (indeed, F Z F = F). Indeed, although in the countable case, the net of groups in [2] can be chosen to be a sequence, not every countable dimensional ordered vector spaces with Riesz interpolation is the limit of a sequence of ordered vector spaces (F n, F n 0). Theorem 2 of [6] characterizes when the net from [6, Theorem 1] can be chosen to be a sequence: exactly when the positive cone is countably generated. Using [2], one sees that an obviously sufficient condition for (V, V + ) to be the limit of a sequence of ordered vector spaces of the form (F n, F n 0) is that (2.1) (V, V + ) = (G, G + ) Z (F, F 0 ). This is the case whenever F = Q. [6, Proposition 5] also shows that (2.1) holds when (V, V + ) is simple, since in this case, we can in fact take (G, G + ) to be a rational vector space. Also, (2.1) holds in the finite

4 4 AARON TIKUISIS rank case, as [7, Theorem 3.2 and Corollary 6.2] likewise show that we can take (G, G + ) to be a rational vector space. However, Theorem 1.1 improves on this result in the finite rank case, by showing that the finite stages have an even more special form their dimension does not exceed the dimension of the limit. 3. Outline of the proof In light of the concrete description above of the inductive limit of ordered vector spaces, saying that (V, V + ) (where dim F V = n) can be realized as an inductive limit of a system (F n, F n 0) φ 2 i (F n, F n 0) is equivalent to saying that there exist linear transformations φ i : F n V such that: (i) φ i is invertible for all i; (ii) V + = φ i (F n 0); and (iii) For all i, φ i (F n 0) φ i+1(f n 0). This idea is used in the proof of Theorem 1.1, which we outline now. We rely on [7] for a combinatorial description of the ordered vector space (V, V + ). Using this description, linear tranformations α ɛ, β R : F n F n are defined for all ɛ, R F >0 := F (0, ). It is shown in Lemma 4.4 that both α ɛ and β R are invertible. In (4.4), we associate to (V, V + ) another ordered vector space (F n, U + ) whose cone is like V + but such that the positive functionals on (F n, U + ) separate the points. We show in Lemma 4.7 (i) and Lemma 4.8 (i), that U + = α ɛ (F n 0), R F >0 and in Lemma 4.7 (ii) and Lemma 4.8 (ii), that V + = ɛ F >0 β R (U + ). Although we don t have β R 1 (F n 0)) β R 2 (α ɛ 2 (F n 0)) when R 1 < R 2 and ɛ 1 > ɛ 2, Lemma 4.9 does allow us to extract an increasing sequence from among all the images β R (α ɛ (F 0 )), such that their union is still all of V The proof in detail We begin with a useful matrix inversion formula.

5 ORDERED VECTOR SPACES WITH INTERPOLATION 5 Lemma 4.1. Let J n M n denote the matrix all of whose entries are 1. Then for λ 1/n, I n + λj n is invertible and (I n + λj n ) 1 = I n λ λn + 1 J n. Proof. Using the fact that J 2 = nj, we can easily verify ( (I + λj) I λ ) λn + 1 J = I. The main result of [7] shows that every finite dimensional ordered directed F-vector space with Riesz interpolation looks like F n with an positive cone given by unions of products of F, F >0 and {0}. To fully describe the result, the following notation for such products is quite useful. Notation 4.2. For a partition {1,..., n} = S 1 S k and subsets A 1,..., A k of a set A, define A S 1 1 A S k k = {(a 1,..., a n ) A n : a i A j i S j, j = 1,..., k}. Theorem 4.3. Every finite dimensional ordered directed F-vector space with Riesz interpolation is isomorphic to (F n, V + ) where V + = S S 0 E0 S F E > S >0 F E S, S is a sublattice of 2 {1,...,n} containing and {1,..., n}, and for each S S, we have a partition {1,..., n} = E 0 S E > S E S, where ES 0 = Sc. We also use the notation E S := E0 S E> S. The sets ES 0, E> S, E S satisfy the following conditions: (RV1) Using the notation E S := E> S Sc, for any S 1, S 2 S, E S 1 S 2 = E S 1 E S 2 ; and (RV2) For any S 1, S 2 S, if S 2 S 1 then E > S 2 \S 1. Remark. Corollaries 5.2 and 6.2 of [7] says that, in the cases F = R and F = Q, every V + given as in the above theorem does actually have Riesz interpolation. Moreover, the proof of [7, Corollary 5.2] works for any other field F R. However, the proof of Theorem 1.1 only uses that V + has the form described in the above theorem, and therefore it gives an entirely different proof of [7, Corollary 5.2], that V + has Riesz interpolation (since Riesz interpolation is preserved under taking inductive limits).

6 6 AARON TIKUISIS Proof. This is simply a special case of [7, Theorem 3.2]. Note that (RV2) appears in [7, Theorem 3.2] as: if S 1 S 2 then E S > 2 \S 1. This is equivalent, to (RV2), since if S 2 S 1 then S 1 S 2 S 2, while if S 1 S 2 then of course S 2 S 1. For each i = 1,..., n, define Z i := {S S : i S c }, and P i := {S S : i E S }. Note that i Z c i and i E P i. For ɛ F >0, define functionals α ɛ i : F n F by (4.1) α ɛ i(z 1,..., z n ) := z i + ɛ j Z i z j ; and for R F >0, define functionals βi R : F n F by (4.2) βi R (y 1,..., y n ) := y i R y j. j P i,p i P j Let us denote α ɛ := (α1, ɛ..., αn) ɛ : F n F n and β R := (β1 R,..., βn R ) : F n F n. Then α ɛ is block-triangular, and β R is triangular, as we shall now explain. For indices i and j, we have j Z i if and only if Z i Z j. We therefore label the blocks of 1,..., αn ɛn ) by sets Z S, where the Z th block consists of indices i such that Z i = Z; we shall use B Z to denote this set of indices, i.e. B Z := {i = 1,..., n : Z i = Z}. For β R, note that if i P i then P i P j, and from this it follows that P i is triangular. Lemma 4.4. For all ɛ, R F >0, α ɛ and β R are invertible. Proof. That β R is invertible follows from the fact that it is triangular with 1 s on the diagonal. To show that α ɛ is invertible, as we already noted that it is block-triangular, we need to check that each block is invertible. In matrix form, the Z th block of α R is equal to I BZ + ɛj BZ, and by Lemma 4.1, this block is invertible. Notation 4.5. For x F n, let us use S x to denote the smallest set S S such that S contains {i = 1,..., n : x i 0}. Lemma 4.6. Let ɛ, R F >0 be scalars and let z F n. Then S z = S α ɛ (z) = S β R (α ɛ (z)).

7 ORDERED VECTOR SPACES WITH INTERPOLATION 7 Proof. To show that S α ɛ (z) S z, it suffices to show that αi(z) ɛ = 0 for all i S z, which we show in (a). Likewise we show in (b) that z i = 0 for all i S α ɛ (z), in (c) that βi R (α ɛ (z)) = 0 for all i S α ɛ (z), and in (d) that αi(z) ɛ = 0 for all i S β R (α ɛ (z)). (a) If i S z then S z Z i and therefore, for every j Z i we have j S z and so z i = 0. Since αi R (z) is a linear combination of {z j : j Z i }, it follows that αi R (z) = 0. (b) We shall prove this by induction on the blocks B Z, iterating Z S in a nonincreasing order. Since i S α R (z) if and only if Z i S α R (z), we only need to consider Z S 1. For a block Z S α R (z) and an index i B Z, we have (4.3) 0 = αi(z) ɛ = z i + ɛ z j + ɛ z j. j B Z j:z j Z By induction, we have that z j = 0 for all j satisfying Z j Z; that is to say, the last term in (4.3) vanishes. Hence, the system (4.3) becomes 0 = (I BZ + ɛj BZ )(z i ) i BZ ; and by Lemma 4.1, it follows that z i = 0 for all i B Z, as required. (c) For (c) and (d), let us set y := α ɛ (x). If i S y then again, S y Z i and so y i = 0 for all j Z i P i. Since β R i (y) is a linear combination of {y i } {y j : j P i }, β R i (y) = 0. (d) If i S β R (y) then we have 0 = β R i (y) = y i R j P i,p i P j y j. As above, j P i implies that j S β R (y). Hence, if we iterate the indices i S c β R (y) in a nondecreasing order of the sets P i then induction proves y i = 0 for all i S β R (y). Our proof makes use of the following positive cone: (4.4) U + := S S F S > 0 Sc. Lemma 4.7. Let R, ɛ F >0 be scalars. Then: (i) α ɛ (F n 0) U +, and (ii) β R (U + ) V +. Proof. (i) Let z F n 0. By Lemma 4.6, we know that α ɛ Z i i i S z. Let us show that α ɛ Z i i that α ɛ (z) U +. For i S z, we have (z) = 0 for (z) > 0 for i S z, from which it follows α ɛ Z i i (z) = z i + ɛ j Z i z j ;

8 8 AARON TIKUISIS so evidently α ɛ Z i i (z) 0 and α ɛ Z i i (z) = 0 would imply that z j = 0 for all j Z i. But if that were the case, then we would have S z Z i, and in particular, i S z, which is a contradiction. Hence α ɛ Z i i (z) > 0. (ii) Let y U +. Then we must have y i > 0 for all i S y. By Lemma 4.6, we already know that βi R (y) = 0 for all i Sy c = ES 0 y. Thus, we need only show that βi R (y) > 0 for i E S > y. For such an i, we have βi R (y) = y i R y j. j P i,p j P i By (i), we know that y i > 0. Since i E S > y, we have P i S y. Therefore if j P i then j S y and so y j = 0. Thus, we in fact have βi R (y) = y i > 0. Lemma 4.8. Let U + be as defined in (4.4). Then: (i) U + = ɛ F >0 ɛ F >0,ɛ <ɛ α(ɛ Z ) (F n 0). (ii) V + = R F >0 R F,R >R βr (U + ). Proof. (i) Let y U +. Define m := min{ y i : i S y } > 0 and M := max{ y i : i S y }, and suppose that ɛ F >0 is such that ɛ < m 2nM, for all Z. Let us show that z = (α ɛ ) 1 (y) satisfies z i 0 for all i. We will show, by induction on the blocks B Z (iterating Z S in a nonincreasing order), that 0 z i M for all i B Z. By the definition of S y, we already know that this holds for Z S y (for if i S y then z i = 0 for i S y ). For i B Z S y, set C i := z i + ɛ z j = y i ɛ z j. j B Z and Then we have By Lemma 4.1, we have j Z,Z j Z C i m ɛnm > m m/2 = m/2 z i = C i On the one hand, this gives and on the other, it gives C i M. ɛ nɛ + 1 j B Z C j. z i > m/2 ɛnm = m/2 m/2 = 0, z i C i M,

9 ORDERED VECTOR SPACES WITH INTERPOLATION 9 as required. (ii) Let x V +. For R F >0 let us denote y R = (y1 R,..., yn R ) := (β R ) 1 (x). For all i S x we already know that yi R = 0 for all i. Moreover, for all i E S > x and all R, we have x i = yi R R yi R ; j P i,p j P i but note that if j P i S x then j S x, and therefore we have yi R = x i > 0. We will show by induction that, for each i ES x there exists R i F >0 such that for all R R R i, we have y R i > y R i > 0. We iterate the indices i in a nonincreasing order of P i. For the index i, we have (4.5) y R i = x i + R j P i,p j P i y R j. If we require that R max{r j : P j P i } then, by induction, we know that yj R 0 for all j P i. Moreover, since i E S x, this means that S x P i and therefore by (RV2) in Theorem 4.3, there exists some j 0 E S > x \P i. Notice that P j S x so that yj R does appear as a summand in the right-hand side of (4.5). Thus, we have y R i = x i + R j P i, P j P i y R j x i + Ry R j 0 = x i + Rx j. Since x j > 0, there exists R = R i for which the right-hand side is positive, and so yi R > 0. Since yj R is a nondecreasing function of R for all j for which P j P i, it is clear from (4.5) that so is yi R. Lemma 4.9. Let R 1, ɛ 1 F >0 be scalars. For any R > R 1, there exist R 2, ɛ 2 F >0 with R 2 > R and ɛ 2 < ɛ 1 such that β R 1 (F n 0)) β R 2 (α ɛ 2 (F n 0)). Proof. Let e 1,..., e n be the canonical basis for F n, so that F n 0 is the cone generated by e 1,..., e n. Then for each of i = 1,..., n, we have by Lemma 4.7 that β R 1 (e i )) V + ; and thus by Lemma 4.8 (i), there exists R 2 > R such that (β R 2 ) 1 (β R 1 (e i ))) U + for all i = 1,..., n. By Lemma 4.8 (ii), there then exists ɛ 2 < ɛ 1 such that ) 1 ((β R 2 ) 1 (β R 1 (e i )))) F n 0

10 10 AARON TIKUISIS for all i = 1,..., n, which is to say, β R 1 (e i )) β R 2 (α ɛ 2 (F n 0)). Since F n 0 is the cone generated by e 1,..., e n, it follows that as required. β R 1 (F n 0)) β R 2 (α ɛ 2 (F n 0)), Proof of Theorem 1.1. Let R 1, ɛ 1 F >0, and, using Lemma 4.9, inductively construct sequences (R i ), (ɛ i ) F >0, such that R i, ɛ i 0 and for each i, β R i (α ɛ i (F n 0)) β R i+1 (α ɛ i+1 (F n 0)). Set φ i = β R i α ɛ i : F n F n. By Lemma 4.8, we have V + = i=1 φ i(f n 0). Our inductive system is thus (F n, F n 0) φ 1 2 φ 1 (F n, F n 0) φ 1 3 φ 2 ; as explained in Section 2, the inductive limit is (F n, φ i (F n 0)) = (V, V + ), as required. i=1 5. Consequences Corollary 5.1. Let (V, V + ) be an n-dimensional ordered directed Q- vector space with Riesz interpolation. Then there exists an inductive system of ordered groups (Z n, Z n 0) whose inductive limit is (V, V + ). φ 2 1 (Z n, Z n 0) Proof. By Theorem refmainresult, let (Q n, Q n 0) φ 2 i (Q n, Q n 0) be an inductive system whose limit is (V, V + ). Since positive scalar multiplication gives an isomorphism of any ordered vector space, we may replace any of the connecting maps with a positive scalar multiples, and still get (V, V + ) in the limit. Hence, we may assume without loss of generality that φ i+1 i (Z n ) Z n. Then, letting φ i+1 i = φ i+1 i Z n, we have an inductive system (Z n, Z n 0) φ 2 i (Z n, Z n 0), whose limit (G, G + ) satisfies (G, G + ) Z (Q, Q + ) = (V, V + ).

11 ORDERED VECTOR SPACES WITH INTERPOLATION 11 Now, we may easily find an inductive system (Z, Z 0 ) ψ 2 1 (Z, Z 0 ) ψ 3 2 whose limit is (Q, Q 0 ). (Such an inductive system necessarily has ψ i+1 i given by multiplication by a positive scalar N i ; and the limit is (Q, Q 0 ) as long as every prime occurs as a root of infinitely many N i.) Thus, by [5, Lemma 2.2], (V, V + ) is the inductive limit of (Z n, Z n 0) Z (Z, Z 0 ) φ2 1 Zψ1 2 (Z n, Z n 0) Z (Z, Z 0 ) φ3 2 Zψ2 3, which is what we require, since (G, G + ) Z (Z, Z 0 ) = (G, G + ) for any ordered abelian group (G, G + ). Corollary 5.2. Let (G, G + ) be a rank n ordered directed free abelian group with Riesz interpolation. Then there exists an inductive system of ordered groups (Z n, Z n 0) φ 2 1 (Z n, Z n 0) whose inductive limit is (G, G + ) Z (Q, Q 0 ). Proof. This follows immediately, as (G, G + ) Z (Q, Q 0 ) is an n-dimensional ordered directed Q-vector space with Riesz interpolation. References [1] Garrett Birkhoff. Lattice, ordered groups. Ann. of Math. (2), 43: , [2] Edward G. Effros, David E. Handelman, and Chao Liang Shen. Dimension groups and their affine representations. Amer. J. Math., 102(2): , [3] Edward G. Effros and Chao Liang Shen. Dimension groups and finite difference equations. J. Operator Theory, 2(2): , [4] K. R. Goodearl. Partially ordered abelian groups with interpolation, volume 20 of Mathematical Surveys and Monographs. American Mathematical Society, Providence, RI, [5] K. R. Goodearl and D. E. Handelman. Tensor products of dimension groups and K 0 of unit-regular rings. Canad. J. Math., 38(3): , [6] David Handelman. Real dimension groups. Working document. arxiv math.ra/ , February [7] Gregory R. Maloney and Aaron Tikuisis. A classification of finite rank dimension groups by their representations in ordered real vector spaces. J. Funct. Anal., 260(11): , [8] Norbert Riedel. A counterexample to the unimodular conjecture on finitely generated dimension groups. Proc. Amer. Math. Soc., 83(1):11 15, Aaron Tikuisis,

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