The PageRank algorithm used by Google expresses the ranking of a web page in terms of two components:

• a base value, (1 - d), usually 0.15
• d * &sum_{i &isin links} PR_i / n_i where PR_i is the page rank of a page that links to this page, and n_i is the number of links from that page.

The PageRank values can be approximated by relaxation by using this formula repeatedly within MapReduce. Each page is initially given a PageRank of 1.0; the sum of all values will always equal the number of pages.

• Map: Share the love: each page distributes its PageRank equally across the pages it links to.

• Reduce: Each page sums the incoming values, multiplies by 0.85, and adds 0.15 .

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