pagerank - 國立臺灣大學
TRANSCRIPT
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PageRankHung-yi Lee
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Search
http://incomebully.com/does-pr-pagerank-still-matter/
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http://www.hobo-web.co.uk/google-pr-update/
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PageRank
• Information of 2008
Source: http://terrylogin.blogspot.com/2008/09/pagerank.html
Rank:8Google台灣首頁:www.google.com.twYoutube台灣首頁:http://tw.youtube.com台灣大學網站首頁:www.ntu.edu.tw
Rank:7痞客邦首頁:www.pixnet.net104人力銀行:www.104.com.tw無名小站首頁:www.wretch.cc
Rank:6博客來網站:www.books.com.tw聯合新聞網首頁:http://udn.com天下雜誌網站首頁:www.cw.com.tw
Rank:5推推王網站:http://funp.com愛情公寓網站:www.i-part.com.twKKman網站首頁:www.kkman.com.tw
Rank:4工頭堅部落格:http://worker.bluecircus.net白木怡言部落格:www.yubou.twRO仙境傳說網站:http://ro.gameflier.com
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PageRank
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PageRank
• Webpages with a higher PageRank are more likely to appear at the top of Google search results.
• PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page’s value.
• Google interprets a link from page A to page B as a vote, by page A, for page B.
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Importance
很多網頁連到
很多網頁連到 被重要網頁連到
hyperlink
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PageRank
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Importance - Formulas
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𝑥1
𝑥2
𝑥3
𝑥4
𝑥1 = 𝑥3 +1
2𝑥4
𝑥2 =1
3𝑥1
𝑥3 =1
3𝑥1 +
1
2𝑥2 +
1
2𝑥4
𝑥4 =1
3𝑥1 +
1
2𝑥2
Consider a random surfer
1/2
1/2
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Importance - Formulas
𝑥1 = 𝑥3 +1
2𝑥4
𝑥2 =1
3𝑥1
𝑥3 =1
3𝑥1 +
1
2𝑥2 +
1
2𝑥4
𝑥4 =1
3𝑥1 +
1
2𝑥2
𝐴𝑥 = 𝑥
The solution x is in the eigenspace of eigenvalue 𝜆 = 1
𝑥 =
𝑥1𝑥2𝑥3𝑥4
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Importance - Formulas
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2
3
4
𝑥1
𝑥2
𝑥3
𝑥4
12
4
9
6
𝐴𝑥 = 𝑥
𝑥 =
𝑥1𝑥2𝑥3𝑥4
The solution x is in the eigenspace of eigenvalue 𝜆 = 1
𝑆𝑝𝑎𝑛 12 4 9 6 𝑇
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Eigenvalue = 1
Column-stochastic Matrix
Column-stochastic matrix always have eigenvalue 𝜆 = 1
How about the Dangling nodes (只入不出)?
Proof
𝐴𝑥 = 𝑥
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Unique Ranking?
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3
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𝑥1
𝑥2
𝑥3
𝑥4
12
4
9
6
Having eigenvalue 𝜆 = 1
The dimension of the subspace is must 1
Unique Ranking
Unique Scoreconstraint
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Unique Ranking?
Dim for 𝜆 = 1 is 2
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2
3
4
5
1/2
1/2
0
0
0
1
2
3
4
50
0
0
1/2
1/2
Any linear combination is in the eigenspace
Basis:
Not Unique Ranking
How about dimension > 1
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Unique Ranking?All entries are 1/n
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There are two ways to surf the web
Follow the link random
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5
Prob 1 – m: Prob m:
Can it be non-uniform?
0.15
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Unique Ranking?
• Unique ranking
• For M, the dim of the eigenvalue 𝜆 = 1 is 1
M is Column-stochastic matrix and “positive”
Proof
Hint: For M, the eigenvectors for eigenvalue 𝜆 = 1 are all “positive” or “negative”
Dim = 1
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Power method
Find 𝑥∗, such that 𝑥∗ = 𝑀𝑥∗, 𝑥∗ 1 = 1
Start from 𝑥0, 𝑥0 1= 1
𝑥1 = 𝑀𝑥0
𝑥2 = 𝑀𝑥1
𝑥𝑘 = 𝑀𝑥𝑘−1…
…
If 𝑘 → ∞
𝑥𝑘 = 𝑥∗
M is very large
Proof
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Power method - Example
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2
3
4
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Actually ……
• The Last Toolbar Pagerank Update was December 2013
• Google declared thereafter: “PageRank is something that we haven’t updated for over a year now, and we’re probably not going to be updating it again going forward, at least the Toolbar version.“