dual is used to extract the dual variable related to a constraint.
Z = dual(F)
After solving an optimization problem we can, e.g., extract the dual variable of the 2nd constraint.
A = [-1 2;-3 -4]; P = sdpvar(2,2); F = [P >= 0, A'*P+P*A <= 0, trace(P) == 1]; optimize(F); Z2 = dual(F(2));
If a constraint has been tagged, we can use the tag to index the constraint.
F = [P >= 0, (A'*P+P*A <= 0):'Lyapunov', trace(P) == 1]; optimize(F); Z2 = dual(F('Lyapunov'));