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A Semantical Analysis of Focusing and Contraction in Intuitionistic Logic

Avellone A., Fiorentini C., Momigliano A.

Fundamenta Informaticae

2015

Vol. 140, nr 3/4

247--262

Focusing is a proof-theoretic device to structure proof search in the sequent calculus: it provides a normal form to cut-free proofs in which the application of invertible and non-invertible inference rules is structured in two separate and disjoint phases. Although stemming from proofsearch considerations, focusing has not been thoroughly investigated in actual theorem proving, in particular w.r.t. termination. We present a contraction-free (and hence terminating) focused multisuccedent sequent calculus for propositional intuitionistic logic, which refines the G4ip calculus in the tradition of Vorob’ev, Hudelmeier and Dyckhoff. We prove completeness of the calculus semantically and argue that this offers a viable alternative to other more syntactical means.

A class of contractions in Hilbert space and applications

Dungey N.

Bulletin of the Polish Academy of Sciences. Mathematics

2007

Vol. 55, no 2

347-355

We characterize the bounded linear operators T in Hilbert space which satisfy T = βI + (1 - β)S where β ∈ (0,1) and S is a contraction. The characterizations include a quadratic form inequality, and a domination condition of the discrete semigroup (T[sup]n)n=1,2,... by the continuous semigroup (e[sup]-t(I-T)t≥o. Moreover, we give a stronger quadratic form inequality which ensures that sup{n||T[sup]n - T[sup]n+1 ||: n = 1, 2,...} < ∞. The results apply to large classes of Markov operators on countable spaces or on locally compact groups.