A Successful Broad-band Survey for Giant Lya Nebulae I: Survey Design and Candidate Selection

Giant Lya nebulae (or Lya "blobs") are likely sites of ongoing massive galaxy formation, but the rarity of these powerful sources has made it difficult to form a coherent picture of their properties, ionization mechanisms, and space density. Systematic narrow-band Lya nebula surveys are ongoing, but the small redshift range covered and the observational expense limit the comoving volume that can be probed by even the largest of these surveys and pose a significant problem when searching for such rare sources. We have developed a systematic search technique designed to find large Lya nebulae at 2<z<3 within deep broad-band imaging and have carried out a survey of the 9.4 square degree NOAO Deep Wide-Field Survey (NDWFS) Bootes field. With a total survey comoving volume of ~10^8 h^-3_70 Mpc^3, this is the largest volume survey for Lya nebulae ever undertaken. In this first paper in the series, we present the details of the survey design and a systematically-selected sample of 79 candidates, which includes one previously discovered Lya nebula.Comment: Accepted to ApJ after minor revision; 25 pages in emulateapj format; 18 figures, 3 table

Underspecification, Inherent Nondeterminism and Probability in Sequence Diagrams

Abstract. Nondeterminism in specifications may be used for at least two different purposes. One is to express underspecification, which means that the specifier for the same environment behavior allows several alterna-tive behaviors of the specified component and leaves the choice between these to those responsible for implementing the specification. In this case a valid implementation will need to implement at least one, but not nec-essarily all, alternatives. The other purpose is to express inherent nonde-terminism, which means that a valid implementation needs to reflect all alternatives. STAIRS is an approach to the compositional and incremental development of sequence diagrams supporting underspecification as well as inherent nondeterminism. Probabilistic STAIRS builds on STAIRS and allows probabilities to be included in the specifications. Underspecifica-tion with respect to probabilities is also allowed. This paper investigates the use of underspecification, inherent nondeterminism and probability in sequence diagrams, the relationships between these concepts, and how these are expressed in STAIRS and probabilistic STAIRS.

A More Complete Model of Communicating Processes

AbstractA previous paper by Hoare gives axioms and proof rules for communicating processes that provide a calculus of total correctness. This paper gives explicit definitions of communicating processes as predicates. The former axioms and proof rules become theorems, proved using the explicit definitions. The defining predicates are more powerful than the proof rules for reasoning about processes, but less often useful for their construction. An implementation of the processes using partial recursive functions is given

A design-based model of reversible computation

Abstract. We investigate, within the UTP framework of designs, the effect of seeing computation as an essentially reversible process. We describe the theoretical link between reversibility and the minimum power requirements of a computation, and we review Zuliani’s work on Reversible pGCL which brings reversible computing within the scope of formal software development. We propose an alternative formalisation of reversible computing which exploits reversibility to provide backtracking. To obtain a basic backtracking language able to search for a single result we exploit the already recognised properties of non-deterministic choice, using it as provisional choice rather than implementor’s choice. We add a “prospective values ” formalism which can describe programs that return all the possible results of a search, and we show how to formally describe the premature termination of such a search, a mechanism analogous to the “cut ” of Prolog. An appendix describes some aspects of the wp calculus in terms of designs, as needed for our proofs. Support for the programming structures described has been incorporated in a reversible virtual machine running under i386 Linux