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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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Sec. 5] Cont<strong>in</strong>uity of Functions 37<br />

2. Po<strong>in</strong>ts of discont<strong>in</strong>uity of a function. We say that a function /(x)has<br />

a discont<strong>in</strong>uity 'at x=* (or at the po<strong>in</strong>t X Q) with<strong>in</strong> the doma<strong>in</strong> of def<strong>in</strong>ition<br />

of the function or on the boundary of this doma<strong>in</strong> if there is a break <strong>in</strong> the<br />

cont<strong>in</strong>uity of the function at this po<strong>in</strong>t.<br />

Example 2. The function f(x)= (Fig. 10 a) is discont<strong>in</strong>uous<br />

when x=l. This function is not def<strong>in</strong>ed at the po<strong>in</strong>t x 1, and no matter<br />

1-2<br />

1 2<br />

how we choose the number /(I), the redef<strong>in</strong>ed function / (x) will not be cont<strong>in</strong>uous<br />

for *=1.<br />

If the function f (x) has f<strong>in</strong>ite limits:<br />

Hm /(*) = f(* -0) and Urn /(*) = /(<br />

and not all three numbers f(x ), /(* ) f (x + Q) are equal, the nx Q is called<br />

a discont<strong>in</strong>uity of the first k<strong>in</strong>d. In particular, if<br />

then * is called a removable discont<strong>in</strong>uity.<br />

For cont<strong>in</strong>uity of a function f(x) at a po<strong>in</strong>t JC Q , it is necessary and sufficient<br />

that

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