Generalized Schwinger Mass Formula
  We generalize Schwinger's original mass formula to the case of an additional isosinglet mixing with the nonet mesons. We then make further generalization to either (i) an arbitrary number of additional isosinglets mixing with nonet mesons, or (ii) an arbitrary number of mesons mixing with an additional isosinglet. In the former case, we present an explicit relation, while in the latter case, we show by numerical example that the new mass formula is only weakly affected by the inclusion of additional mesons, and hence holds with good accuracy for each of the 3x3 mass sub-matrices. 
Leonid Burakovsky, Philip R. Page, Terry Goldman