Saharon Shelah

  1. Clones above the unary clone.

    Authors: Saharon Shelah, Martin Goldstern, Gábor Sági
    Subjects: Rings and Algebras
    Abstract

    Let c be the cardinality of the continuum.

    We give a family of pairwise incomparable clones (on a countable base set)
    2^c members, all with the same unary fragment, namely the set of all unary
    operations.

    We also give, for each n, a family of 2^c clones all with the same n-ary
    fragment, and all containing the set of all unary operations.

  2. Pseudo PCF.

    Authors: Saharon Shelah
    Subjects: Logic
    Abstract

    We continue our investigation on pcf with weak form of choice.
    Characteristically we assume DC + P(Y) when looking and prod_{s in Y} delta_s.
    We get more parallel of theorems on pcf.

  3. Inner product space with no ortho-normal basis without choice.

    Authors: Saharon Shelah
    Subjects: Logic
    Abstract

    We prove in ZF that there is an inner product space, in fact, nicely
    definable with no orthonormal basis.

  4. Hereditary Zero-One Laws for Graphs.

    Authors: Saharon Shelah, Mor Doron
    Subjects: Logic
    Abstract

    We consider the random graph M^n_{\bar{p}} on the set [n], were the
    probability of {x,y} being an edge is p_{|x-y|}, and \bar{p}=(p_1,p_2,p_3,...)
    is a series of probabilities. We consider the set of all \bar{q} derived from
    \bar{p} by inserting 0 probabilities to \bar{p}, or alternatively by decreasing
    some of the p_i. We say that \bar{p} hereditarily satisfies the 0-1 law if the
    0-1 law (for first order logic) holds in M^n_{\bar{q}} for any \bar{q} derived
    from \bar{p} in the relevant way described above.

  5. The Stationary Set Splitting Game.

    Authors: Saharon Shelah, Paul Larson
    Subjects: Logic
    Abstract

    The \emph{stationary set splitting game} is a game of perfect information of
    length $\omega_{1}$ between two players, \unspls and \spl, in which \unspls
    chooses stationarily many countable ordinals and \spls tries to continuously
    divide them into two stationary pieces. We show that it is possible in ZFC to
    force a winning strategy for either player, or for neither.

  6. PCF arithmetic without and with choice.

    Authors: Saharon Shelah
    Subjects: Logic
    Abstract

    We deal with relatives of GCH which are provable. In particular we deal with
    rank version of the revised GCH. Our motivation was to find such results when
    only weak versions of the axiom of choice are assumed but some of the results
    gives us additional information even in ZFC.

  7. Large continuum, oracles.

    Authors: Saharon Shelah
    Subjects: Logic
    Abstract

    Our main theorem is about iterated forcing for making the continuum larger
    than aleph_2. We present a generalization of math.LO/0303294 which is dealing
    with oracles for random, etc., replacing aleph_1, aleph_2 by lambda,lambda^+
    (starting with lambda=lambda^{<lambda}>aleph_1). Well, instead of properness we
    demand absolute c.c.c. So we get, e.g. the continuum is lambda^+ but we can get
    cov(meagre)=lambda. We give some applications.

  8. A dichotomy for the number of ultrapowers.

    Authors: Ilijas Farah, Saharon Shelah
    Subjects: Logic
    Abstract

    We prove a strong dichotomy for the number of ultrapowers of a given
    countable model associated with nonprincipal ultrafilters on N. They are either
    all isomorphic, or else there are $2^{2^{\aleph_0}}$ many nonisomorphic
    ultrapowers. We prove the analogous result for metric structures, including
    C*-algebras and II$_1$ factors, as well as their relative commutants and
    include several applications. We also show that the C*-algebra B(H) always has
    nonisomorphic relative commutants in its ultrapowers associated with
    nonprincipal ultrafilters on N.

  9. Maximal failures of sequence locality in a.e.c.

    Authors: Saharon Shelah
    Subjects: Logic
    Abstract

    We are interested in examples of a.e.c. with amalgamation having some
    (extreme) behaviour concerning types. Note we deal with k being sequence-local,
    i.e. local for increasing chains of length a regular cardinal. For any cardinal
    theta>= aleph_0 we construct an a.e.c. with amalgamation k with L.S.T.(k) =
    theta, |tau_K| = theta such that {kappa : kappa is a regular cardinal and K is
    not (2^kappa, kappa)-sequence-local} is maximal. In fact we have a direct
    characterization of this class of cardinals: the regular kappa such that there
    is no uniform kappa^+-complete ultrafilter.

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