By Roger Godement
Research quantity IV introduces the reader to useful research (integration, Hilbert areas, harmonic research in crew idea) and to the tools of the speculation of modular capabilities (theta and L sequence, elliptic services, use of the Lie algebra of SL2). As in volumes I to III, the inimitable sort of the writer is recognizable right here too, not just due to his refusal to put in writing within the compact sort used these days in lots of textbooks. the 1st half (Integration), a smart mix of arithmetic stated to be 'modern' and 'classical', is universally important while the second one half leads the reader in the direction of a really energetic and really good box of analysis, with very likely large generalizations.
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Extra info for Analysis IV: Integration and Spectral Theory, Harmonic Analysis, the Garden of Modular Delights (Universitext)
Example text
By assigning once and for all an arbitrary value to it since, if (fn ) is a countable family with values in [−∞, +∞], the set of x where −∞ and +∞ appear in the sequence (fn (x)) is measurable as it is the intersection of the measurable sets fn−1 ({−∞}) ∩ fn−1 ({+∞}) . The product of two complex-valued measurable functions is measurable. This result holds for any continuous bilinear map. For example, if f and g take values in a separable Hilbert space, the function x → (f (x)|g(x)) is measurable.
Let (fn ) be a sequence of measurable functions with values in a separable metric space P and converging ae. to a function f : X −→ P . For all > 0 and all integrable sets A ⊂ X, there exists an integrable set B ⊂ A such that µ(A − B) < and in which the sequence (fn (x)) converges uniformly to f (x). 46 XI – Integration and Fourier Transform Removing a null set from A, we may suppose that lim fn (x) = f (x) for all x ∈ A without exception. Henceforth, all arguments apply exclusively to A. For integers m, p ≥ 1, consider the measurable set Am (p) = {d [f (x), fn (x)] < 1/m for all n ≥ p} .
1) The following properties are immediate restatements of results from the previous n◦ . An open set U is integrable if and only if µ∗ (U ) < +∞, and then µ(U ) = µ∗ (U ) (n◦ 4, lemma 3). The intersection of a finite or countable family of integrable sets is integrable (n◦ 5, lemma 3) . 2) µ An = inf µ(An ) = lim µ(An ) . The union of a finite family of integrable sets is integrable (n◦ 5, lemma 1) . 3) µ An ≤ µ(An ) since the characteristic function of A is bounded above by the sum of those of the sets An .