Subtraction Piece in the Cqtti~g~ari. F.prmula. for the p·n · Mass-Difference ·,, ·

We discuss the · possible absence of the J=O ·and 1=1 fixed-pole and A 2 Regge-pole terms in the longitudinaJ part of the P'n difference of the forW-ard virtual Ctimptoii amplitude, JT L• at least in the Bjorken scaling limit. We write down an unsubtracted dispersion relation for JTL to show that the condition for the Cottingham forrriulli. to converge is R'"'-'q2/2y2 in the · scaling liinit. We estimate the d~ep inelastic contribUtion to the pen mass difference by use' of the recent experimental · data and find it too sinall ·to. be able i:o reverse 'the wrong. sign uriless a parameter appearing in. fhe expression for R is large negative.· · § 1.. 'Introduction and s~mary Though Cottingham's representatimi1> foi the electromagnetic mass difference of hadrons is quite remarkable iii e:x:pressing _ the _mass differe~ce by physically observable quantities alone, there have been so~e.. diffi~ultles. in calculati:p.g the JI = 1 mass difference by this.representation ~ The forward. vir.tual Compton am