The commutator of X1 with X2
The commutator of the third extensions of X1 and X2 involves four elements.
The generators G1 and G2 associated to the lists X1 and X2
If G1 and G2 are differential operators (mappings), the commutator of G1 and G2 is returned as a mapping as well:
The commutator of the third extensions of G1 and G2