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Real and Complex Analysis (Rudin)

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138 REAL AND COMPLEX ANALYSIS<br />

7.S Weak I! Iff E I!(Rk) <strong>and</strong> A > 0, then<br />

(1)<br />

because, putting E = {I f I > A}, we have<br />

Am(E) ~ [If I dm ~ [ If I dm = IIfIIl'<br />

JE<br />

JRk<br />

Accordingly, any measurable functionffor which<br />

(2)<br />

A . m{ I f I > A} (3)<br />

is a bounded function of A on (0, 00) is said to belong to weak I!.<br />

Thus weak I! contains I!. That it is actually larger is shown most simply by<br />

the function l/x on (0, 1).<br />

We associate to each fE I!(Rk) its maximal function Mf: Rk~ [0,00], by<br />

setting<br />

(Mf)(x) =<br />

sup _(1) [ I f I dm.<br />

O

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