  This paper is devoted to integrable gl(n|m) spin chains which allow for
formulation of the string hypothesis. Considering the thermodynamic limit of
such spin chains, we derive linear functional equations that symmetricaly treat
holes and particles. The functional equations naturally organize different
types of excitations into a pattern equivalent to the one of Y-system, and, not
surprisingly, the Y-system can be easily derived from the functional equations.
The Y-system is known to contain most of the information about the symmetry of
the model, therefore we map the symmetry knowledge directly to the description
of string excitations. Our analysis is applicable for highest weight
representations which for some choice of the Kac-Dynkin diagram have only one
nonzero Dynkin label. This generalizes known results for the AdS/CFT spectral
problem and for the Hubbard model.
