On Day 18, we returned to the spacetime symmetries we discussed last time, but treated them with a bit more rigor and tighter notation. In the context of the Newton’s 2^{nd} law, we worked out the math for

**Translation group**as applied to three-dimensional space and to time.- Rotations in space and their matrix representation in 2D.
- We formally defined the
**orthogonal group**O(2) and the**special orthogonal group**SO(2).