TheoremBase

The Sequences k ↦ fk(p)f_k(p) of a Sequence in ℝ^I, and the Map of Their Limits

For a sequence of real-valued maps on a set I, each evaluation at a point of I is a real sequence, and if all these sections converge there is exactly one map in ℝ^I to which they converge pointwise, in either framework of convergence.

Statement

In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let II be a set, let RI\mathbb{R}^{I} be the set of maps from II to R\mathbb{R} of Sets and Maps: Ordinary Notation §maps, let dRd_{\mathbb{R}} be the distance on R\mathbb{R} of Metric Spaces: Balls, the Metric Topology, and the Real and Complex Numbers as Metric Spaces §real-line, and let (fk)k∈N(f_{k})_{k\in\mathbb{N}} be a sequence in RI\mathbb{R}^{I}.

For every p∈Ip\in I, the map k↦fk(p)k\mapsto f_{k}(p) on N\mathbb{N}, written (fk(p))k(f_{k}(p))_{k}, is a sequence in R\mathbb{R}, with kk-th term fk(p)f_{k}(p).

If for every p∈Ip\in I the sequence (fk(p))k(f_{k}(p))_{k} is convergent in (R,dR)(\mathbb{R},d_{\mathbb{R}}), then there is exactly one L∈RIL\in\mathbb{R}^{I} such that, for every p∈Ip\in I, (fk(p))k(f_{k}(p))_{k} converges to L(p)L(p) in (R,dR)(\mathbb{R},d_{\mathbb{R}}); and L(p)L(p) is the limit lim⁡k→∞fk(p)\lim_{k\to\infty}f_{k}(p) in (R,dR)(\mathbb{R},d_{\mathbb{R}}) for every p∈Ip\in I.

If for every p∈Ip\in I the sequence (fk(p))k(f_{k}(p))_{k} is convergent in the sense of Convergent Sequences of Real Numbers §converges, then there is exactly one L∈RIL\in\mathbb{R}^{I} such that, for every p∈Ip\in I, (fk(p))k(f_{k}(p))_{k} converges to L(p)L(p) in that sense; this LL is the map of the clause limit-map, and L(p)L(p) is the limit lim⁡k→∞fk(p)\lim_{k\to\infty}f_{k}(p) of The Limit of a Convergent Sequence §limit for every p∈Ip\in I.

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