Compatible associative products and trees

We compute dimensions of graded components for free algebras with two compatible associative products, and give a combinatorial interpretation of these algebras in terms of planar rooted trees.Comment: 19 pages, added a note on relation to other operad

Freeness theorems for operads via Gr\"obner bases

We show how to use Groebner bases for operads to prove various freeness theorems: freeness of certain operads as nonsymmetric operads, freeness of an operad Q as a P-module for an inclusion P into Q, freeness of a suboperad. This gives new proofs of many known results of this type and helps to prove some new results.Comment: 15 pages, no figures, corrected typos and changed in parts the structure of the pape

Pattern avoidance in labelled trees

We discuss a new notion of pattern avoidance motivated by the operad theory: pattern avoidance in planar labelled trees. It is a generalisation of various types of consecutive pattern avoidance studied before: consecutive patterns in words, permutations, coloured permutations etc. The notion of Wilf equivalence for patterns in permutations admits a straightforward generalisation for (sets of) tree patterns; we describe classes for trees with small numbers of leaves, and give several bijections between trees avoiding pattern sets from the same class. We also explain a few general results for tree pattern avoidance, both for the exact and the asymptotic enumeration.Comment: 27 pages, corrected various misprints, added an appendix explaining the operadic contex

The third parafermionic chiral algebra with the symmetry Z_{3}

We have constructed the parafermionic chiral algebra with the principal parafermionic fields \Psi,\Psi^{+} having the conformal dimension \Delta_{\Psi}=8/3 and realizing the symmetry Z_{3}.Comment: 6 pages, no figur

Spin--spin critical point correlation functions for the 2D random bond Ising and Potts models

We compute the combined two and three loop order correction to the spin-spin correlation functions for the 2D Ising and q-states Potts model with random bonds at the critical point. The procedure employed is the renormalisation group approach for the perturbation series around the conformal field theories representing the pure models. We obtain corrections for the correlations functions which produce crossover in the amplitude but don't change the critical exponent in the case of the Ising model and which produce a shift in the critical exponent, due to randomness, in the case of the Potts model. Comparison with numerical data is discussed briefly.Comment: 10pp, latex, PAR--LPTHE 94/1

Anick-type resolutions and consecutive pattern avoidance

For permutations avoiding consecutive patterns from a given set, we present a combinatorial formula for the multiplicative inverse of the corresponding exponential generating function. The formula comes from homological algebra considerations in the same sense as the corresponding inversion formula for avoiding word patterns comes from the well known Anick's resolution.Comment: 16 pages. Preliminary version, comments are welcom