# Noncrossing partitions under rotation and reflection

- Published in 2005
- Added on

In the collections

We consider noncrossing partitions of [n] under the action of (i) the reflection group (of order 2), (ii) the rotation group (cyclic of order n) and (iii) the rotation/reflection group (dihedral of order 2n). First, we exhibit a bijection from rotation classes to bicolored plane trees on n edges, and consider its implications. Then we count noncrossing partitions of [n] invariant under reflection and show that, somewhat surprisingly, they are equinumerous with rotation classes invariant under reflection. The proof uses a pretty involution originating in work of Germain Kreweras. We conjecture that the "equinumerous" result also holds for arbitrary partitions of [n].

## Links

## Other information

- key
- Noncrossingpartitionsunderrotationandreflection
- type
- article
- date_added
- 2018-10-27
- date_published
- 2005-10-09

### BibTeX entry

@article{Noncrossingpartitionsunderrotationandreflection, key = {Noncrossingpartitionsunderrotationandreflection}, type = {article}, title = {Noncrossing partitions under rotation and reflection}, author = {David Callan and Len Smiley}, abstract = {We consider noncrossing partitions of [n] under the action of (i) the reflection group (of order 2), (ii) the rotation group (cyclic of order n) and (iii) the rotation/reflection group (dihedral of order 2n). First, we exhibit a bijection from rotation classes to bicolored plane trees on n edges, and consider its implications. Then we count noncrossing partitions of [n] invariant under reflection and show that, somewhat surprisingly, they are equinumerous with rotation classes invariant under reflection. The proof uses a pretty involution originating in work of Germain Kreweras. We conjecture that the "equinumerous" result also holds for arbitrary partitions of [n].}, comment = {}, date_added = {2018-10-27}, date_published = {2005-10-09}, urls = {http://arxiv.org/abs/math/0510447v3,http://arxiv.org/pdf/math/0510447v3}, collections = {Combinatorics,Easily explained,Geometry,Things to make and do}, url = {http://arxiv.org/abs/math/0510447v3 http://arxiv.org/pdf/math/0510447v3}, year = 2005, urldate = {2018-10-27}, archivePrefix = {arXiv}, eprint = {math/0510447}, primaryClass = {math.CO} }