homology ring of loop space is group ring with loop space of universal cover

Proposition

Let 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_fb201ba05b721383a59fd39671361d3d4e3b0683.svg be an anima and

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_ad512e23790aa4ae0b203d3a9a21b9ffb8249f58.svg

be a fiber sequence for the universal cover.

Then the homology ring of 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_08653953bd74b7528e0f0b0c024183b171718607.svg is given by the group ring

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_fc16d9a25fe7ec13c4734dc9ec28ea0e931ff401.svg

Proof

using loop space is a coproduct of the loop space of the universal cover we get

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_64fc80a596a3b6cf01f0febbe030eda15c037be8.svg

This shows the additive isomorphism

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_5e4f6830ea6436b17fafeaa1c1c7f5bc430b12da.svg

Now we want to identify the multiplication:
Here under the above equivalence 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_49eca95ec4c6adb54498e6e5bf682fa1b6e66a3e.svg the connected corresponding to 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_3ba7c526a3f4108df0e4561cc1e900af1dcdd30b.svg acts (under the identification of 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_9d4ef611aab24f45df4acea940fb9e88703b084f.svg used by connected components of an anima valued group equivalent) by mapping the 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_9d593f4cb7ea70257451c18d99ea40c1b6070b89.svg-th component to the 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_87c8b279538462d6334109c7204618f32f9c0fc8.svg resp. 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_bc060cf4a2017a1fba83fa3dfb4a95a06662be92.svg-th component - depending whether we multiply from the left or the right.

Thus

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_c06f6d6df20972dd31dd725cf2d158a150eb5ddc.svg

Therefore we get the multiplication

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_880187ca0f8263b72cd23b9995a6087beee23f96.svg

and analogously 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_c0d694f43724e231ebf7f734c79c383d19144326.svg

Furthermore the inclusion

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_e663272500abb5d42595e0b23d878bc3da34495b.svg

is a map of loop spaces, hence 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_e254c0195710607e3a968d0a6a75f61334ae126a.svg is a map of groups.

Thus the multiplication of 20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_bb143da75848d911ce8c2859e3e342c41abb4334.svg since

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_e602b423580e16689389034620bd8139726aa25b.svg

is map of graded rings (and inclusion of connected components).

Therefore we get in general

20251212-homology_ring_of_loop_space_is_group_ring_with_loop_space_of_universal_cover_2c8d951f1aadc817fd3b12f7f994d7c089d88b66.svg

which is precisely the presentation of a group ring :)

Date: nil

Author: Anton Zakrewski

Created: 2025-12-12 Fr 13:01