Unitary group Updated +Created
Complex analogue of the orthogonal group.
One notable difference from the orthogonal group however is that the unitary group is connected "because" its determinant is not fixed to two disconnected values 1/-1, but rather goes around in a continuous unit circle. is the unit circle.
Unitary matrix Updated +Created
Applications:
Why are complex numbers used in the Schrodinger equation? Updated +Created
Video 1.
Necessity of complex numbers in the Schrödinger equation by Barton Zwiebach (2017)
Source.
This useless video doesn't really explain anything, he just says "it's needed because the equation has an in it".
The real explanation is: they are not needed, they just allow us to write the equation in a shorter form, which is always a win: physics.stackexchange.com/questions/32422/qm-without-complex-numbers/557600#557600