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  1. http://www.nd.edu/~cholak/papers/tarragona.pdf
  2. http://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.ndjfl/1040136917
  3. http://news.sina.com.cn/c/2012-03-22/022424153423.shtml
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