Hello, @Pascal4QM ! How are you? Well, excuse me, in fact, the correct expression is D[mu](phi_(X))=d_[mu](phi_(X))+gA[mu](n_&x (phi_(X))). The metric is Minkowski (+--) in 2+1 dimensional space-time. You can consider phi as iso-vector with three components phi[1], phi[2] and phi[3]. The iso-vector n is unity with components n[1], n[2] and n[3]. The covariant derivative was redefining for A[mu]->A[mu]/g.

How can I obtain the field equations for a Skyrme Field Theory using the Maple?