ERRATUM:

The proof of Theorem 1 (page 9) is not correct.
The correct proof can be found in the following publication:

Sakai, T.:
On the Task of Finding One Highly Relevant Document with High Precision,
IPSJ Transactions Vol.47, No.SIG 4 (TOD29), pp.13-27, 2006.
Available at:
http://www.jstage.jst.go.jp/article/ipsjdc/2/0/174/_pdf