Invertible matrices. Or if you think a bit more generally, an invertible linear map.
Non invertible are excluded "because" otherwise it would not form a group (every element must have an inverse).
- Important Lie groups | 0, 978, 14
- Lie group | 233, 2k, 24
- Differential geometry | 12, 2k, 25
- Geometry | 0, 2k, 34
- Mathematics | 17, 13k, 336
- Ciro Santilli's Homepage | 262, 182k, 3k
- Special linear group | 18