where 7 comes from (2) and 'f = (7f,.., 7 J) where 7f is the sth element of vector — (Ir(s)SfIr(s))~1 Ir(s)s(pf)∙
Remark that out of equilibrium retail prices can be obtained from observed equilibrium retail
prices, retail margins at equilibrium and out of equilibrium retail margins using : pfr(s) = 7fr(s) —
(ps — ws — cs)+ ps where 7fr(s) = pfr(s) — ws — cs is the out of equilibrium retail margin∙ Moreover,
pʃ can be deduced from the differentiation of the retailer’s first order conditions with respect to
wholesale prices∙ These first order conditions are, for all r = 1,.., R and VJ ∈ Sr,
Sj (pfr )+ ∑ ' — ^s — Cs) ' .' =0
sCSr ∖Gfr Spj
which gives for r = 1,.., R, J ∈ Sr and s = 1,.., J1
^ ∂sj (pfrjfS) ∂pfrj
S-~f Sffrjjl ∂ws
IC[1,..,J }∖Gfr °pl s
1 ∂ss (7fr(j)) + v Ssi(p-fr(j)) ⅜fω
[sGSr} dpfr(j ^S dpfr(j) ∂ws
1J l^ sr j j
+ Σ
lCSr∖Gfr
(pfr — wι — cι) ∑
sG[1,..,J }\Gfr
a2sl(p'fr(j)) ap'fr(j)
∂pfjrω∂pfsrj dws
(16)
Defining Spf the matrix (J x J) of the second derivatives of the market shares with respect to
retail prices whose element (s, I) is
) ∙ p
affr(f)affr(f) , ∙ ∙
/
Sp ≡
J2f
∖
a2s1 (pfr<f>)
apfr(f)apfr(f)
∙
∙
∙
∂2s1 (pfrt-f~>)
∂pfrωdpfjrf
∂2sj (pfrt-^) ∖
apf,r<f>apfr<f>
∂2sj (ffr<f>)
∂pfr-f∂pjr-f /
we can write equation (16) in matrix form to obtain
FW [sf + irSfp' + (spfIr7fr∣...∣sp;Ir7fr)] Ir — IrSp (ιr — Ir) = 0
where 7fr = pfr — w — c∙
Denoting Mfr the matrix ^S~ + IrSf' + (SpfIr7fr∣...∣SpfjIr7fr)] we can solve this system of
equations and get the following expression for J2W

_
f∑r-1 Ir Mf r Ir Sf (Ir
i)) (∑Π1
Ir Mf r Mfr Ir
Equation (15) shows that one can express the manufacturer’s price-cost margins vector as depen-
ding on the demand function and the structure of the industry by replacing the expression for
Ff
^ W ∙
17
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