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This survey considers multiple orthogonal polynomials with respect to Nikishin systems generated by a pair (sigma(1), sigma(2)) of measures with unbounded supports (supp(sigma(1)). R+, supp(sigma(2)) subset of R-) and with sigma(2) discrete. A Nikishin-type equilibrium problem in the presence of an external field acting on R+ and a constraint on R- is stated and solved. The solution is used for deriving the contracted zero distribution of the associated multiple orthogonal polynomials. Bibliography: 56 titles.
hermite-pade approximants; multiple orthogonal polynomials; orthogonality with respect to a discrete measure; weak asymptotics; vector equilibrium problem; nikishin systems