A. R Bergen and V. Vittal, Power systems analysis, Prentice-Hall, (1999).
Motivated by this observation, define the reduced vector of active powers as pr(x) = pij(x) (i, j) G , and consider the View PDF
 A. R. Bergen and V. Vittal, Power Systems Analysis, 2nd ed.
1Sobolev Institute of Mathematics, Pr. View PDF