An analog to the Gleason theorem for measures on logics of projections in indefinite metric spaces is proved.
IntroductionThe problem of describing measures on logics is well-known [B]. A celebrated theorem of Gleason [GI serves to be a base for the Quantum Measure Theory. The theorem asserts every nonnegative measure p on the orthogonal projections of a Hilbert space K with dim K 2 3 to be of the form p ( p ) = t r (Tp), where T 2 0 is a uniquely determined trace class operator. Here we give an analogy to the Gleason's theorem for a real measure on an indefinite metric space.We present the necessary definitions and notation. Let H be a space with an indefinite metric [ a , a ] , a canonical decomposition H = H+ [+I H-, and with a canonical symmetry J. In the terminology of [AI] H is a K r e h space (= J-space). H is a Hilbert space with respect to the inner product (see [AI]) (2, z ) = [Jz, 4. There exist orthogonal projections Q+ and Qsuch that Q++Q-= I , J = Q+-Q-, Q + H = H+, Q-H = Hwith [z, z] = (Jz, z ) , for any z, z E H. Let S = {z E H : (2, z) = 1). Put I?+ = {z E H : [z,z] = 1) and I?-= {x E H : [z,z] = -1). The set r = r+ uris an indefinite analogy to the unit sphere S. Let B(H)P be the set of all orthogonal projections in B ( H ) and P be the set of all J-selfadjoint projections in B ( H ) , i. e., P = { p E B ( H ) : p2 = p , [p., z] = [x,pz], for any z, z E H} It is easy to see that any one-dimensional projection p E P can be represented in the form p f = [ f, f ] [ . , f l f , f E r . With respect to the ordering, p 5 q if and 1991 Mathematics Subject Classification. Primary 81 P 10, 46 L 50, 46 B 09, 46 C 20; Secondary Keywords and phrases. Quantum logics, measure, indefinite metric, W* -algebra. 28 A 80. 230Math. Nachr. 184 (1997) only if pq = qp = p and the orthocomplementation, p -+ pL 5 Ip , the set P is a quantum logic. Now let P+(P-) be the set of all projections p E P , for which the subspace pH is positive (for all x E pH, x # 0, [x, x] > 0) (respectively, negative, i. e., for all x E pH, x # 0, [x,x] < 0). Any projection e E P is representable in the form e = e+ + e-, where e+ E P+, e-E P-. Definition 1.1. A mapping p : P -+ IR is called a measure if p ( e ) = C p ( e L ) for any representation e = C e , (where eLea = 0, L # (Y and the sum is understood in the strong sense). A measure p is said to be indefinite if p / p + 1 0 and p / p -5 0;bounded if cfi = sup { lp(p)l IIpII-' : p E P } < m.
The main resultsOur main result is the following: Theorem 2.1. Let Y : P + R be a measure on an infinite-dimensional Kreiiz space H , dim H+ 5 dim H-. Then there exist a unique J -selfadjoint trace -class operator T and a unique number c such that (2.1) v(e) = tr(Te) + cdim(etH), for all e E P . Moreover, if dim H+ = 00, then c = 0 (0m 0 ) .Remark 2.2. An indefinite measure is an analog to a probability measure for the logic P of the J-selfadjoint projections. In [MI, we proved that for any indefinite measure p in a Kre'in space, dim H 2 3, the formula (2.1) is true.At first we prove the followin...