1,432 research outputs found
Hopf algebras of endomorphisms of Hopf algebras
In the last decennia two generalizations of the Hopf algebra of symmetric
functions have appeared and shown themselves important, the Hopf algebra of
noncommutative symmetric functions NSymm and the Hopf algebra of quasisymmetric
functions QSymm. It has also become clear that it is important to understand
the noncommutative versions of such important structures as Symm the Hopf
algebra of symmetric functions. Not least because the right noncommmutative
versions are often more beautiful than the commutaive ones (not all cluttered
up with counting coefficients). NSymm and QSymm are not truly the full
noncommutative generalizations. One is maximally noncommutative but
cocommutative, the other is maximally non cocommutative but commutative. There
is a common, selfdual generalization, the Hopf algebra of permutations of
Malvenuto, Poirier, and Reutenauer (MPR). This one is, I feel, best understood
as a Hopf algebra of endomorphisms. In any case, this point of view suggests
vast generalizations leading to the Hopf algebras of endomorphisms and word
Hopf algebras with which this paper is concerned. This point of view also sheds
light on the somewhat mysterious formulas of MPR and on the question where all
the extra structure (such as autoduality) comes from. The paper concludes with
a few sections on the structure of MPR and the question of algebra retractions
of the natural inclusion of Hopf algebras of NSymm into MPR and section of the
naural projection of MPR onto QSymm.Comment: 40 pages. Revised and expanded version of a (nonarchived) preprint of
200
Niceness theorems
Many things in mathematics seem lamost unreasonably nice. This includes
objects, counterexamples, proofs. In this preprint I discuss many examples of
this phenomenon with emphasis on the ring of polynomials in a countably
infinite number of variables in its many incarnations such as the representing
object of the Witt vectors, the direct sum of the rings of representations of
the symmetric groups, the free lambda ring on one generator, the homology and
cohomology of the classifying space BU, ... . In addition attention is paid to
the phenomenon that solutions to universal problems (adjoint functors) tend to
pick up extra structure.Comment: 52 page
When is negativity not a problem for the ultra-discrete limit?
The `ultra-discrete limit' has provided a link between integrable difference
equations and cellular automata displaying soliton like solutions. In
particular, this procedure generally turns strictly positive solutions of
algebraic difference equations with positive coefficients into corresponding
solutions to equations involving the "Max" operator. Although it certainly is
the case that dropping these positivity conditions creates potential
difficulties, it is still possible for solutions to persist under the
ultra-discrete limit even in their absence. To recognize when this will occur,
one must consider whether a certain expression, involving a measure of the
rates of convergence of different terms in the difference equation and their
coefficients, is equal to zero. Applications discussed include the solution of
elementary ordinary difference equations, a discretization of the Hirota
Bilinear Difference Equation and the identification of integrals of motion for
ultra-discrete equations
Attractive internal wave patterns
This paper gives background information for the fluid dynamics video on
internal wave motion in a trapezoidal tank.Comment: 2 pg, movie at two resolutions _low(Low-resolution) and
_hr(High-resolution
A universal isomorphism for p-typical formal groups and operations in Brown-Peterson cohomology
AbstractWe construct an abstract isomorphism of p-typical formal groups which is universal for isomorphisms of p-typical formal groups over Z(p)-algebras or characteristic zero rings. Associated to this universal isomorphism is a homomorphism of rings Z[V1, V2, …] → Z[V1, V2, …; T1, T2, …] which (after localization at p) can be identified with the right unit homomorphism ηR: BP∗(pt) → BP∗(BP) of the Hopf-algebra BP∗(BP) of Brown-Peterson (co)homology. We calculate ηR modulo the ideal (T1, T2, …)2. These results are then used to obtain information on some of the operations of Brown-Peterson cohomology
Attractive internal wave patterns
This paper gives background information for the fluid dynamics video on
internal wave motion in a trapezoidal tank.Comment: 2 pg, movie at two resolutions _low(Low-resolution) and
_hr(High-resolution
- …