Abstract. An important problem in web search is to determine the importance of each page. From the mathematical point of view, this problem consists in finding the nonnegative left eigenvector of a matrix corresponding to its dominant eigenvalue 1. Since this matrix is neither stochastic nor irreducible, the power method has convergence problems. So, the matrix is replaced by a convex combination, depending on a parameter c, with a rank one matrix. Its left principal eigenvector now depends on c, and it is the PageRank vector we are looking for. However, when c is close to 1, the problem is ill-conditioned, and the power method converges slowly. So, the idea developed in this paper consists in computing the PageRank vector for several values of c, and then to extrapolate them, by a conveniently chosen rational function, at a point near 1. The choice of this extrapolating function is based on the mathematical expression of the PageRank vector as a function of c. Numerical experiments end the paper.
The problemThe mathematical problem behind web search is the computation of the nonnegative left eigenvector of a p × p matrix P corresponding to its dominant eigenvalue 1, where p is the number of pages in Google (8.06 billion at the end of March 2005). Since P is not stochastic (some rows of P may contain only zeros due to the so-called dangling nodes), it is replaced by the matrix P = P + dw T with w ∈ R p a probability vector, that is, such that w ≥ 0 and (w, e) = 1 with e = (1, . . . , 1) T , and d = (d i ) ∈ R p the vector with d i = 1 if deg(i) = 0, and 0 otherwise, where deg(i) is the outdegree of the page i, that is, the number of pages it points to.Since the matrix P is not irreducible, it is replaced by the matrixwhere c is a parameter between 0 and 1, and E = ev T with e = (1, . . . , 1) T ∈ R p and v is a probability vector. Such a modification of the matrix corresponds to adding to all pages a new set of outgoing transitions with small probabilities. The probability distribution given by the vector v can differ from a uniformly distributed vector, and the resultant PageRank can be biased to give preference to certain kinds