We know the Lorentz group is O(3,1) in 4 dimensional spacetime.
We know that there are 4 disconnected components in the Lorentz group O(3,1), and https://math.stackexchange.com/questions/2204349/difference-between-the-lorentz-group-and-the-restricted-lorentz-group
π0(O(1,3))≅Z2×Z2.
My question is that in QFT we have a discrete charge conjugation symmetry C, parity P and time reversal T.
We know the P and T flips the 4 disconnected components of O(3,1) into each other.
But the charge conjugation C seems not to be in the Lorentz group. Is the group generated by CPT and the Lorentz group O(3,1) the group O(1,3))×Z2?
where the new Z2 is a charge conjugation C symmetry?
How do we understand the total group of CPT and the Lorentz group O(3,1)?