ON THE STAR DELTA TRANSFORMATION IN NETWORK RELIABILITY

Lehman observed that star-delta and delta-star transformation cannot a lways be applied to networks with perfect (unfailing) vertices so as t o exactly preserve reliability, and Rosenthal and Frisque introduced i mperfect vertices to make exact delta-star transformations possible. W e give an explicit condition that is satisfied if and only if a networ k with perfect vertices can be subjected to an exact star-delta or del ta-star transformation, and we discuss the introduction of imperfect f aces to make exact star-delta transformations possible.