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IPO Auctions: English, Dutch, ... French, and Internet

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32 BIAIS AND FAUGERON-CROUZET<br />

The investor with a bad signal is better off bidding for S shares than for ¯Q if<br />

c > ¯Q<br />

N−1<br />

<br />

<br />

S<br />

πl[v(l) − p]<br />

.<br />

S(1 − k + N − (l + 1)) + (l + 1) ¯Q<br />

l=0<br />

Denote K1 the right-h<strong>and</strong> side of this inequality.<br />

Now consider the informed investor with a good signal. She is better off bidding<br />

for ¯Q than for S shares if<br />

That is, if<br />

N−1<br />

<br />

<br />

¯Q<br />

S<br />

πl[vl+1 − p]<br />

− c<br />

S(1 − k + N − (l + 1)) + (l + 1) ¯Q<br />

l=0<br />

<br />

N−1<br />

S<br />

> S πl[vl+1 − p]<br />

.<br />

S(1 − k + N − l) + (l) ¯Q<br />

l=0<br />

N−1<br />

πl[vl+1 − p]S<br />

l=0<br />

<br />

¯Q<br />

×<br />

S(1 − k + N − l) + l ¯Q + ¯Q −<br />

<br />

S<br />

> c.<br />

S(1 − k + N − l) + l ¯Q + S<br />

Denote K2 the left-h<strong>and</strong> side of the inequality. Note that it is indeed positive,<br />

since<br />

¯Q<br />

S(1 − k + N − l) + l ¯Q + ¯Q ><br />

S<br />

S(1 − k + N − l) + l ¯Q + S .<br />

Proof of Proposition 3. To establish that the strategies stated in the proposition<br />

form an equilibrium, we need to prove that the investor with a good signal, anticipating<br />

that the other investors follow the equilibrium strategy, prefers to purchase<br />

S/(N + 1) shares at price p ¯ , rather than bidding more aggressively, to increase her<br />

market share. Indeed, if she does not want to do so, then a fortiori, the uninformed<br />

investor or the informed investor with bad news do not want to undercut, while<br />

they are still willing to buy a constant amount at p ¯ .<br />

Anticipating that the other investors follow their equilibrium strategies, she faces<br />

the residual supply curve:<br />

<br />

S<br />

S − N<br />

N + 1 − σ (p − p ¯ )<br />

<br />

.

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