By Luc Tartar

After publishing an creation to the Navier–Stokes equation and oceanography (Vol. 1 of this series), Luc Tartar follows with one other set of lecture notes according to a graduate direction in components, as indicated via the name. A draft has been to be had on the net for many years. the writer has now revised and polished it right into a textual content available to a bigger audience.

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**Extra info for An Introduction to Sobolev Spaces and Interpolation Spaces**

**Sample text**

He worked in Catania, in Genova (Genoa), in Torino (Turin), Italy, and after 1939 in New York, NY. For a measurable function f , the deﬁnition of support must be diﬀerent than the one for continuous functions, because a function is an equivalence class of functions equal almost everywhere, and the deﬁnition of the support of f must not change if one changes f on a set of measure 0. For an open set ω one says that f = 0 on ω if f is equal to 0 almost everywhere in ω; if for a family ωi , i ∈ I, of open sets such that one has f = 0 on ωi for each i ∈ I, then f = 0 on ω = i∈I ωi , and in the case where I is not countable one may use a partition of unity for showing that the support of f is the closed set whose complement is the largest open set on which f = 0.

For 1 ≤ p < ∞, and any integer m ≥ 0, the space Cc∞ (RN ) is dense in W m,p (RN ). , θn (x) = θ1 nx , with θ1 ∈ Cc∞ (RN ), 0 ≤ θ(x) ≤ 1 on RN and θ(x) = 1 for |x| ≤ 1. For u ∈ W m,p (RN ), one deﬁnes un = θn u, and one notices that un → u in W m,p (RN ) strong as n → ∞. Indeed, one has |un (x)| ≤ |u(x)| almost everywhere, and un (x) → u(x) as n → ∞, and by the Lebesgue dominated convergence theorem one deduces that un → u in Lp (RN ) strong as n → ∞. Then for |α| ≤ m one has β α−β u, and the term for β = 0 converges to Dα u Dα un = β≤α α β D θn D again by the Lebesgue dominated convergence theorem, while the terms for |β| > 0 contains derivatives of θn which converge uniformly to 0, so that one has Dα un → Dα u in Lp (RN ) strong as n → ∞.

2 The Lebesgue Measure, Convolution 13 f2 ∈ Cc (R) with R f2 (x) dx = 0, and let f3 be the Heaviside18 function, deﬁned by f3 (x) = 0 for x < 0 and f3 (x) = 1 for x > 0; one sees immediately that f1 f2 = 0 and f4 = f2 f3 ∈ Cc (R), and one has to check that f2 can be chosen in such a way that R f4 (x) dx = 0; if one chooses f2 with support +1 +1 in [−1, +1], with −1 f2 (y) dy = 0 and −1 (1 − y)f2 (y) dy = 0, then one has x +1 f4 (x) = −1 f2 (y) dy, and R f4 (x) dx = −1 (1 − y)f2 (y) dy = 0. , measurable functions which are integrable on every compact, denoted by L1loc (RN ), then for f, g ∈ L1loc (RN ) one can deﬁne the convolution product f g if the following condition is satisﬁed: for every compact C ⊂ RN there exist compact sets A, B ⊂ RN such that x ∈ support(f ), y ∈ support(g) and x + y ∈ C imply x ∈ A, y ∈ B.