Hermitian symmetric space Sp (n)/U (n) to derive new integrable systems with unitary
invariance. These systems consist of a bi-Hamiltonian modified Korteweg-de Vries equation
and a Hamiltonian sine-Gordon (SG) equation, involving a scalar variable coupled to a
complex vector variable. The Hermitian structure of the symmetric space Sp (n)/U (n) is used
in a natural way from the beginning in formulating a complex matrix representation of the …