▁but ▁measures ▁such ▁as ▁the ▁P SI ▁and ▁the ▁R SI ▁identify ▁the ▁" p iv otal ness " ▁of ▁companies ▁to ▁meeting ▁the ▁demand ▁on ▁the ▁system . ▁A ▁company ▁that ▁is ▁very ▁p iv otal ▁can ▁be ▁said ▁to ▁have ▁a ▁potential ▁for ▁high ▁market ▁power . ▁ ▁Emp ir ical ▁estimates ▁in ▁electric ity ▁mark ets ▁have ▁related ▁the ▁R SI ▁to ▁price ▁cost ▁marg ins . ▁ ▁P iv otal ▁Supp lier ▁Index ▁( P SI ) ▁The ▁P iv otal ▁Supp lier ▁Index ▁cons iders ▁the ▁demand ▁in ▁addition ▁to ▁the ▁supply ▁side . ▁At ▁the ▁most ▁basic ▁level , ▁the ▁P SI ▁answers ▁the ▁question ▁if ▁a ▁particular ▁supp lier , ▁the ▁" p iv otal ▁supp lier ", ▁is ▁needed ▁to ▁supply ▁the ▁demand : ▁would ▁it ▁be ▁possible ▁to ▁supply ▁the ▁pre val ent ▁demand ▁also ▁without ▁this ▁supp lier ▁or ▁is ▁he ▁mand atory ? ▁ ▁The ▁P iv otal ▁Supp lier ▁Index ▁is ▁expressed ▁as ▁the ▁following : ▁ ▁P SI ▁= ▁I [ C x ▁> ▁ Σ ni = I ▁Ci ▁- ▁Total ▁Cons umption ] ▁ ▁where ▁C x ▁is ▁the ▁capacity ▁of ▁the ▁potential ▁p iv otal ▁supp lier ▁and ▁the ▁sum ▁of ▁Ci ▁the ▁capacity ▁of ▁all ▁supp liers . ▁The ▁function ▁I [ .] ▁is ▁the ▁indicator ▁function , ▁which ▁takes ▁the ▁value ▁ 1 ▁if ▁the ▁expression ▁" ." ▁contained ▁in ▁it ▁is ▁true . ▁ ▁An ▁equivalent ▁expression ▁is ▁the ▁following : ▁ ▁P SI