
doi: 10.1007/bf02567991
In her thesis [Commun. Algebra 23, No. 1, 219-243 (1995)], \textit{Maria Pia Solèr}, a student of the late H. Gross, proved that for every infinite dimensional hermitian space \((E, \langle\;\rangle)\) over a skew field \(K\) which is orthomodular, i.e. every subspace \(X\) of \(E\) satisfies \[ \text{if } X= (X^\perp )^\perp \quad \text{then} \quad E= X\oplus X^\perp, \tag{1} \] and which contains an orthonormal sequence \((e_n )_{n\in \mathbb{N}}\), necessarily \(K\) is \(\mathbb{R}\), \(\mathbb{C}\) or \(\mathbb{H}\) and \((E, \langle\;\rangle)\) is a Hilbert space over \(K\). In [\textit{H. A. Keller}, \textit{U. M. Künze} and \textit{M. P. Solèr}, `Orthomodular spaces' (to appear)] a simplification for the case of \(K\) being commutative is given. The aim of this paper is to present another simplification for the commutative case which carries over to the non-commutative case almost literally (see Section 5). The theorem we are going to prove first in the commutative case reads as follows: Theorem. Let \(K\) be a field, \({}^*: K\to K\) an involution on \(K\), \(E\) an infinite dimensional \(K\)-vector space, and \(\langle\;\rangle: E\times E\to K\) a hermitian form on \(E\). Then the only cases in which \((E,\langle\;\rangle)\) is orthomodular and contains an infinite orthonormal sequence \((e_n )_{n\in \mathbb{N}}\) occur if \((K,{}^*)= (\mathbb{R}, id)\) or \((\mathbb{C}, {}^-)\) and \((E,\langle\;\rangle)\) is a Hilbert space over \((K, {}^*)\). Although the proof given here is considerably shorter than the original one, it still uses the main ideas of M. P. Solèr.
510.mathematics, Characterizations of Hilbert spaces, orthomodular, Article
510.mathematics, Characterizations of Hilbert spaces, orthomodular, Article
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