Abstract. We study the rate of convergence of interpolating simultaneous rational approximations with partially prescribed poles to so called Nikishin systems of functions. To this end, a vector equilibrium problem in the presence of a vector external field is solved which is used to describe the asymptotic behavior of the corresponding second type functions which appear.
Generalized Hermite-Padé approximantsLet ∆ 1 be a bounded interval of the real line R. By M(∆ 1 ) we denote the set of all Borel measures with constant sign (positive or negative) whose support supp(·) is contained in ∆ 1 and contains infinitely many points. Let s = (s 1 , · · · s m ) be a vector of measures belonging to M(∆ 1 ). The Markov function corresponding to the measure s i ∈ M(∆ 1 ) is given byCertainly, s i is a holomorphic function in C \ ∆ 1 . We restrict our attention to a special class of vector measures introduced by E. M. Nikishin in [11] and adopt the notation introduced in [8]. Let σ 1 and σ 2 be two measures supported on R with constant sign and let ∆ 1 , ∆ 2 denote the convex hull of supp(σ 1 ) and supp(σ 2 ) respectively; that is,When it is convenient we use the differential notation of a measure. Then σ 1 , σ 2 is a measure with constant sign and supported on supp(σ 1 ) ⊂ ∆ 1 .Definition 1. Given a system of closed bounded intervals ∆ 1 , . . . , ∆ m satisfying ∆ j−1 ∩ ∆ j = ∅, j = 2, . . . , m, and finite Borel measures σ 1 , . . . , σ m with constant sign and Co(supp(σ j )) = ∆ j , we define inductivelyWe say that S = (