# Operads in Algebra and Topology

*Graduate seminar on Topology (S4D2)*

Semester | Summer 2020 |
---|---|

Organisers |
Andrea Bianchi Florian Kranhold |

Weekly time and place | Wednesdays, 14:15–15:45 Online via Zoom |

Organisational meeting | Thursday, February 6^{th} 2020, 11:00–12:00SR 0.006 |

Documents | Plan of the talks |

### Summary

Sometimes a topological space *X* is endowed with a continuous product
(think for example of a Lie group): in this case one can say a lot about the
topological invariants of *X*. For example, its homology becomes a graded
ring and its fundamental group is abelian. However, many interesting
spaces *X* are endowed with several natural operations with multiple
inputs and one output, and there is no canonical way to choose one
operation. An operad helps us keeping track of all possible operations that we
want to consider: this notion naturally arose in the 1970s from the study of
iterated loop spaces by Boardman, Vogt and May.

In the context of linear algebra, operads classify various types of (multilinear) operations that one can put on a module, and the relations that we want to impose among these operations (think of an algebra structure on a module, with associativity and commutativity as properties).

This seminar aims to provide an introduction to operad theory, with a focus on its applications to algebraic topology. We will in particular see how operads help us understanding the homology of spaces and sequences of spaces, and detect properties of their homotopy type.

One of our main goals will be the *recognition principle* of May,
stating that if *X* has an action of the operad *E _{d}* of
little

*d*-cubes (and assuming that

*X*is grouplike), then

*X*is homotopy equivalent to a

*d*-fold loop space of another space. We will also consider the following situation: we have a space

*X*which naturally decomposes as a disjoint union of spaces

*X*indexed by natural numbers; an operad

_{n}*O*acts on

*X*, and for all

*n*there are natural maps

*X*coming from this action. In this case, the action of

_{n}→ X_{n+1}*O*can help us understand how the different spaces

*X*are interrelated, and obtain some information on the

_{n}*stable homology*of

*X*, i.e. the colimit of

*H*) for

_{∗}(X_{n}*n*going to infinity. We will in particular discuss the

*group completion theorem*for the homology of certain algebras over the operad

*E*.

_{1}Finally, we will consider the surface operad *M* introduced by
Tillmann and we will see that if *X* has an action of an *operad with
homological stability* (for example *M*) and *X* is grouplike,
then *X* is homotopy equivalent to an infinite loop space.

### Plan of the talks

All talks, also the ones at exceptional dates, will take place 14:15–15:45.

Talk title | Date | Speaker | |
---|---|---|---|

1 | H-spaces and d-fold loop spaces | 22.04. | Agata Sienicka |

2 | Symmetric monoidal categories | 24.04. | Janina Bernardy |

3 | Operads and little cubes | 29.04. | Jakub Löwit |

4 | Monads | 06.05. | Nicholas Schwab |

5 | Simplicial objects and bar constructions | 13.05. | Andrea Lachmann |

6 | Two applications of quasifibrations | 20.05. | Andrea Bianchi |

7 | Two applications of the bar construction | 03.06. | Ben Steffan, Jonathan Pampel |

8 | Algebraic operads | 10.06. | Daniel Mulcahy, Christian Kremer |

9 | Homological stability in many examples | 17.06. | Constanze Schwarz, Jonah Epstein |

10 | The surface operad | 24.06. | Malte Kornemann |

11 | The group completion theorem | 01.07. | Branko Juran |

12 | Ω^{∞}-spaces and operads with homological stability | 08.07. | Urs Flock, Robin Louis |

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