Convexity. An analytic viewpoint by Barry Simon

By Barry Simon

Convexity is necessary in theoretical points of arithmetic and likewise for economists and physicists. during this monograph the writer offers a finished perception into convex units and features together with the infinite-dimensional case and emphasizing the analytic viewpoint. bankruptcy one introduces the reader to the elemental definitions and ideas that play crucial roles in the course of the ebook. the remainder of the booklet is split into 4 components: convexity and topology on infinite-dimensional areas; Loewner's theorem; severe issues of convex units and comparable concerns, together with the Krein-Milman theorem and Choquet conception; and a dialogue of convexity and inequalities. The connections among disparate issues are in actual fact defined, giving the reader a radical realizing of the way convexity comes in handy as an analytic software. a last bankruptcy overviews the subject's historical past and explores extra a few of the topics pointed out prior. this is often a superb source for an individual drawn to this principal subject.

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1), if λ > μ, H(λ) ≥ λμ−1 H(μ) > H(μ), so H is strictly monotone, so there is λ0 with H(λ0 ) = 1 and H(λ) > 1 if λ > λ0 . It follows that λ−1 0 = f and QF (f / f ) = H(λ0 ) = 1. (vi) ⇒ (i) We will show that ∼(i) implies ∼(vi). So suppose that F does not obey the Δ2 condition. Let βn = 21/n . 4. It follows that lim sup F (βn x)/F (x) = ∞. 25) ∞ n =1 Suppose that (M, dμ) is nonatomic. Since F (xn ) ≥ F (x1 ) ≥ 1, αn ≤ ∞ 1 −n −1 = 2 . 6, we can find disjoint sets A1 , A2 , . . so n =1 2 μ(An ) = αn .

For x ≥ x0 , F (kx) ≤ B F (x) + ε ≤ (B + 1)F (x) Now pick so k ≥ 2. Then for x ≥ x0 , F (2x) ≤ F (k x) ≤ (B + 1)F (k (i) ⇒ (v) −1 x) ≤ · · · ≤ (B + 1) F (x) Since (D− F )(x) is monotone, if (i) holds and x ≥ x0 , 2x x(D− F )(x) ≤ x D− F (y) dy ≤ F (2x) ≤ C F (x) Orlicz spaces 37 so sup x≥x 0 x(D− F )(x) ≤C F (x) Since sup 1≤x≤x 0 x(D− F )(x) <∞ F (x) (v) holds. (v) ⇒ (iv) Suppose (v) holds. Let the sup be A and let x ≥ 1 and k > 1. 18) Pick k so A(log k) ≤ 12 . 18) implies F (kx) ≤ 2F (x) so (iv) holds.

Pick N0 so n =N 0 αn < μ(A∞ ) and the disjoint An , An +1 , . . and still find f = 1 while QF (f / f ) = 2−N 0 . 1 2 While monotone convergence upwards may not imply convergence in norm, it does imply convergence of the · F norms. 10 Let f ∈ L(F ) (M, dμ) and suppose |fn | is monotone increasing with lim|fn (m)| = |f (m)|. Then (i) f1 F ≤ · · · ≤ fn F ≤ · · · ≤ f F (ii) fn F → f F (iii) For some λ > 0, λfn → λf in mean. (iv) If F obeys the Δ2 condition, |fn | − |f | F → 0. Proof (i) If 0 ≤ |g| ≤ |h|, then QF (λ−1 |g|) ≤ QF (λ−1 |h|) so {λ | QF (λ−1 |g|) ≤ 1} ⊇ {λ | QF (λ−1 |h| ≤ 1} so g F ≤ h F .

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