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where the adjacency function

is 0 if page pj does not link to pi, and normalized such that, for each j, the equation is:

That is, the elements of each column sum up to 1.

This is a variant of the eigenvector centrality measure used commonly in network analysis.

The values of the PageRank eigenvector are fast to approximate (only a few iterations are needed),

and in practice it gives good results.

As a result of Markov theory, it can be shown that the PageRank of a page is the probability of being

at that page after lots of clicks. This happens to equal t ? 1 where t is the expectation of the number

of clicks (or random jumps) required to get from the page back to itself.

The main disadvantage is that it favors older pages, because a new page, even a very good one, will

not have many links unless it is part of an existing site (a site being a densely connected set of pages,

such as Wikipedia). The Google Directory (itself a derivative of the Open Directory Project) allows

users to see results sorted by PageRank within categories. The Google Directory is the only service

offered by Google where PageRank directly determines display order. In Google’s other search serv-

ices (such as its primary Web search) PageRank is used to weight the relevance scores of pages

shown in search results.

Several strategies have been proposed to accelerate the computation of PageRank.

Various strategies to manipulate PageRank have been employed in concerted efforts to improve

search results rankings and monetize advertising links. These strategies have severely impacted the

reliability of the PageRank concept, which seeks to determine which documents are actually highly

valued by the Web community.

Google is known to actively penalize link farms and other schemes designed to artificially inflate

PageRank. How Google identifies link farms and other PageRank manipulation tools are among

Google’s trade secrets.

To say that links are an important part of SEO for your web site is probably an understatement. If

you want your site to be found, you must have links in from other sites. Without those links, you

may as well be an island sitting in the middle of an ocean full of other web sites.

How Links and Linking Work

You’ve seen how important links are to your SEO strategy, and you’ve seen how links affect your

Google PageRank, but how, really, do links work for improving your SEO? As you’ve already seen,

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Understanding the Role of Links and Linking

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