Talk:2.3.7 subgroup

retraction/expansion terminology

I know this is well-defined on temperaments, but for subgroups, I would prefer the more well-known terms restriction/extension.

– Sintel🎏 (talk) 23:18, 22 April 2025 (UTC)

It seems necessary to consider the relationship with Group extension. (Can group extension be used in relation to 2.3.5 and 2.9.5?) ... Oh dear, there's also Retract (group theory).

Retraction seems like it could be directly used in the definition of saturation. --Dummy index (talk) 12:01, 23 April 2025 (UTC)

That's a lot of abstraction for what is essentially very simple: extension = make bigger, restriction = make smaller. – Sintel🎏 (talk) 20:33, 23 April 2025 (UTC)

I asked Claude 3.7 and was convinced that group extension can be used for weak 'extension'. (It's not that I understood cohomology or anything like that.) To return to the topic, I agree to put aside retract, which is similar to both saturation and projection. --Dummy index (talk) 15:00, 24 April 2025 (UTC)