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10.20 Internet Search Engines 707 procedure starting from the new page, and thereby surfs from page to page. She tracks how many times she visits each page in the set after 10, 100, 1000, 10,000, and 20,000 mouse clicks and creates Table 1. (Notice that the number of pages visited is one more than the number of mouse clicks.)Table 1 Number of Visits to Each Page1 0.0000 0.2727 0.2079 0.1648 0.1504 0.1506 2 1.0000 0.1818 0.1584 0.1479 0.1391 0.1395 3 0.0000 0.2727 0.2673 0.2707 0.2706 0.2712 4 0.0000 0.0000 0.0396 0.0999 0.1096 0.1103 5 0.0000 0.1818 0.2178 0.1548 0.1415 0.1372 6 0.0000 0.0909 0.1089 0.1618 0.1889 0.1912 |
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Tiêu đề: |
Internet Search Engines |
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(2)Notice that• The sum of the entries of the ith row of an adjacency matrix is the number of incoming links to the ith page from the other pages.• The sum of the entries in the j th column is the number of outgoing links on the j th page to other pages.We make the following definition |
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Tiêu đề: |
i"th row of an adjacency matrix is the number of incominglinks to the"i"th page from the other pages.• The sum of the entries in the"j"th column is the number of outgoing links on the"j |
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10.20 Internet Search Engines 709 You should be able to see that 0 ≤ b ij ≤ 1 and that the entries in each column of B sum up to 1. As an example, the probability transition matrix associated with (2) isB =⎡⎢ ⎢⎢ ⎢⎢ ⎢⎢ ⎢⎣0 1/3 0 0 1/3 1/30 0 1/3 0 1/3 01 0 0 1/2 0 1/30 1/3 0 0 0 1/30 1/3 1/3 0 0 00 0 1/3 1/2 1/3 0⎤⎥ ⎥⎥ ⎥⎥ ⎥⎥ ⎥⎦ |
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Tiêu đề: |
Internet Search Engines |
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10.20 Internet Search Engines 711 To implement his strategy Bob creates a new probability transition matrix M = [m ij ] in whichm ij = δb ij + 1 − δn (6)with b ij as given in Definition 2. He then replaces (4) with the iterative scheme |
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Tiêu đề: |
M= [m"ij"]inwhich"m"ij =δb"ij"+1−"δ"n" (6)with"b"ij |
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1. Without damping, find the page ranks of the following web- graphs of three pages by determining their normalized eigen- vectors for the eigenvalue 1.112233 (a)(b)Figure Ex-1 2. Show that starting with an initial state vector with equal en-tries in the iterative scheme x (k) = M x (k− 1) is equivalent to averaging the iterates obtained by starting with each of the pages in the webgraph individually |
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a)(b")Figure Ex-12.Show that starting with an initial state vector with equal en-tries in the iterative schemex"(k)=M"x"(k |
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3. Show that if every page in a webgraph is linked to every other page, then all the pages have the same rank for any damping factor δ in [ 0 , 1 ] |
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4. Show that the matrix M in Equation (7) is a transition matrix;that is, its entries are nonnegative and its column sums are all equal to 1. Also show that M can be written as in Equation (8) |
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Tiêu đề: |
M"in Equation (7) is a transition matrix;that is, its entries are nonnegative and its column sums are allequal to 1. Also show that"M |
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(3)This matrix incorporates the probability information for advancing randomly from one page to the next with a mouse click. For example, if we know with certainty that Alice is initially on Page 2, then her initial state vector is |
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