Split-complex numbers: https://en.wikipedia.org/wiki/Split-complex_number
Euler’s formula: https://en.wikipedia.org/wiki/Euler%27s_formula#Proofs
The 3blue1brown video: https://www.youtube.com/watch?v=kYB8IZa5AuE&index=3&list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab
The exp & log graph I mentioned: https://www.desmos.com/calculator/xm1iojttdh