Which implies
δ [Bk + 2c] = k — δ-φ ——c[Bkr + 2Xcr + r2 + σ22
or equivalently
- σu1 ] - 2δr
c = k 1 - b - r 1 - Φ Γχ--ʌ) - 21-φ(xC-^c) [Bkr + 2Xcr + r2 + σ22 - σ2ι]
2 Lδ _l L ∖ σv ) _l 2σv ∖ σv )
Proof ofProposition 6 For all values of parameters B, k, r, δ, and σv, there exists a critical level
Xc such that an increase in the variance σ2u decreases the optimal loading c.
Proof: First, we will show that using the above parameters the optimal loading c = Xc — σv. Using
the proposed c in the equation that implicitly defines c we get
■- φ (⅛
+ɪ φX- )
2σv ∖ σv j
[Bkr + 2Xcr + r2 + ∆σU ]
= Xc - σv - k [■ - B]
+ r [1 - Φ(1)] + -1-φ(1)[Bkr + 2χr + r2 + ∆σU]
2σv
= χ l^σv + r,(1)1 - σv - k Γ1 - BI + r [1 - ф(1)] + J_φ(ι')[Bkr + r2 + ∆σl]
L σv J 2 Lδ J '2σ,∙
kσv [1 - b] + 2σv - 2rσv[1 - ф(1)] - φ I bKr + r2 + ^U] k Γ1 _l 1 2 2
= ----"------------------z--σv - Ô U - b + r [1 - ф(1)] + — ,(1)[Bkr + r + ^U]
2σv 2 δ J 2σv
= k [1 - b∖ + σ - r[1 - Φ(1)] - ɪφ(1)[bKr + r2 + ∆σ^] - σ - k [1 - B + r [1 - Φ(1)] + ɪφ(1)[Bkr + r2 + ∆σ^]
2 δ 2σv 2 δ 2σv
= 0
The first line is the equation that defines c. The second line uses the proposed c = χc — σv. The
third line factors out Xc before the fourth line uses the expression for Xc.
Second, to get ddc^, totally differentiate the above equation that implicitly defines c to obtain
∣1+ r ,, (χc-cʌ - ɪ φ (χc-cʌ [Bkr + 2Xcr + r2 +∆σU]) de
I σv σ σv J 2σv ∖ σv J J
+ I r[Xc2 c] φ (χc---k - y12φ (χc---k [Bkr + 2Xcr + r2 + ∆σ'2] - [X; 3 c] φ' (χc--c^ [Bkr + 2Xcr + r2 + ∆σU]∣ dσv
I σ2 ∖ σv J 2σ2 ∖ σv J 2σ3 ∖ σv J J
=0
31
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