A Note on the Complexity of Restricted Attribute-Value Grammars

The recognition problem for attribute-value grammars (AVGs) was shown to be undecidable by Johnson in 1988. Therefore, the general form of AVGs is of no practical use. In this paper we study a very restricted form of AVG, for which the recognition problem is decidable (though still NP-complete), the R-AVG. We show that the R-AVG formalism captures all of the context free languages and more, and introduce a variation on the so-called `off-line parsability constraint', the `honest parsability constraint', which lets different types of R-AVG coincide precisely with well-known time complexity classes.Comment: 18 pages, also available by (1) anonymous ftp at ftp://ftp.fwi.uva.nl/pub/theory/illc/researchReports/CT-95-02.ps.gz ; (2) WWW from http://www.fwi.uva.nl/~mtrautwe

Levelable Sets and the Algebraic Structure of Parameterizations

Asking which sets are fixed-parameter tractable for a given parameterization constitutes much of the current research in parameterized complexity theory. This approach faces some of the core difficulties in complexity theory. By focussing instead on the parameterizations that make a given set fixed-parameter tractable, we circumvent these difficulties. We isolate parameterizations as independent measures of complexity and study their underlying algebraic structure. Thus we are able to compare parameterizations, which establishes a hierarchy of complexity that is much stronger than that present in typical parameterized algorithms races. Among other results, we find that no practically fixed-parameter tractable sets have optimal parameterizations

Use of reverse transcription-polymerase chain reaction (RT-PCR) for Cymbidium mosaic virus (CyMV) detection in orchids

The reverse transcription-polymerase chain reaction CRT-PCR) was adapted for detection of Cymbidium mosaic virus CCyMV) in orchids. The oligonucleotide primers used were selected from the predicted homologous coat protein region of CyMV and other Potexviruses which enabled to amplify approximately 313 bp and 227 bp fragments using optimum reaction conditions of 2.5 mM MgCh and 30 cycles of amplification. The RT-PCR allowed the detection of CyMV RNA and virion in purified fonns as well as in crude tissue extracts of orchid. Direct CyMV RNA detection was possible in leaves, shoots, stems, roots and petals. The detection limits of RNA in purified CyMV and virion by RT-PCR described were 10 ng and 2 ng, respectively. The PCR amplified fragments were confinned to be CyMV-specific by dotblot hybridization with DIG-labelled CyMV cDNA probe. The suitability of the RT-PCR in routine testing of CyMV was detennined and compared with those of DAS-ELISA. Thirty samples of leaf tissues representing various genera or hybrids of cultivated local orchid from glasshouse and commercial nurseries were tested for CyMV by RT-PCR and DAS-ELISA. Among 15 samples that tested positive for CyMV infection by DAS-ELISA, only 7 samples gave the expected amplification fragments when subjected in RTPCR assays. The equal detection limit on purified CyMV virion by RT-PCR and DAS-ELISA and lower sensitivity of RT-PCR in detecting CyMV in a field indexing trial suggested that RT-PCR is unsuitable to replace DAS-ELISA for routine testing of CyMV in local orchids

Discovering Motifs in Real-World Social Networks

We built a framework for analyzing the contents of large social networks, based on the approximate counting technique developed by Gonen and Shavitt. Our toolbox was used on data from a large forum---\texttt{boards.ie}---the most prominent community website in Ireland. For the purpose of this experiment, we were granted access to 10 years of forum data. This is the first time the approximate counting technique is tested on real-world, social network data

Sparse Selfreducible Sets and Nonuniform Lower Bounds

It is well-known that the class of sets that can be computed by polynomial size circuits is equal to the class of sets that are polynomial time reducible to a sparse set. It is widely believed, but unfortunately up to now unproven, that there are sets in (Formula presented.), or even in (Formula presented.) that are not computable by polynomial size circuits and hence are not reducible to a sparse set. In this paper we study this question in a more restricted setting: what is the computational complexity of sparse sets that are selfreducible? It follows from earlier work of Lozano and Torán (in: Mathematical systems theory, 1991) that (Formula presented.) does not have sparse selfreducible hard sets. We define a natural version of selfreduction, tree-selfreducibility, and show that (Formula presented.) does not have sparse tree-selfreducible hard sets. We also construct an oracle relative to which all of (Formula presented.) is reducible to a sparse tree-selfreducible set. These lower bounds are corollaries of more general results about the computational complexity of sparse sets that are selfreducible, and can be interpreted as super-polynomial circuit lower bounds for (Formula presented.)