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Îñê³ëüêè ðÿä (12.2.7) çá³ãàºòüñÿ, òî ìàºìî<br />
lim σ<br />
= σ<br />
n ,<br />
n →∞<br />
äå σ — ñóìà ðÿäó (12.2.7). Çã³äíî ç óìîâîþ òåîðåìè ïîñë³äîâíîñò³<br />
çð³çàíèõ ñóì ðÿä³â (12.2.6) – (12.2.7) ³ ñóìà ðÿäà<br />
(12.2.7) ïîâ’ÿçàíà íåð³âí³ñòþ<br />
0 ≤ S n ≤σ n ≤σ.<br />
Íà ï³äñòàâ³ òîãî, ùî åëåìåíòè íåâ³ä’ºìí³, ïîñë³äîâí³ñòü S n<br />
íåñïàäíà ³, êð³ì òîãî, çã³äíî ç îñòàííüîþ íåð³âí³ñòþ âîíà º<br />
îáìåæåíîþ. Ó â³äïîâ³äíîñò³ äî òåîðåìè 5.2.6 ïîñë³äîâí³ñòü<br />
ìຠãðàíèöþ.<br />
Òåîðåìó äîâåäåíî.<br />
12.2.3. Îçíàêà çá³æíîñò³ Äàëàìáåðà<br />
Òåîðåìà 12.2.1. Íåõàé óñ³ ÷ëåíè ðÿäó<br />
a 1 + a 2 + a 3 + ... + a n + ... (12.2.8)<br />
äîäàòí³, ³ íåõàé<br />
a<br />
lim<br />
a<br />
n→+∞<br />
n + 1<br />
n<br />
=ρ. (12.2.9)<br />
Òîä³ ïðè ρ < 1 ðÿä çá³ãàºòüñÿ, à ïðè ρ > 1 ðîçá³ãàºòüñÿ.<br />
Ïðè ρ = 1 ïèòàííÿ ïðî çá³æí³ñòü ðÿäó çàëèøàºòüñÿ â³äêðèòèì,<br />
òîáòî ðÿä ∑<br />
∞ a<br />
n ìîæå ÿê çá³ãàòèñÿ, òàê ³ ðîçá³ãàòèñÿ.<br />
n=<br />
1<br />
Ó öüîìó âèïàäêó íåîáõ³äíå äîäàòêîâå äîñë³äæåííÿ çà äîïîìîãîþ<br />
³íøèõ îçíàê.<br />
Äîâåäåííÿ. Çà îçíà÷åííÿì ãðàíèö³ ïîñë³äîâíîñò³<br />
(äèâ. ï. 5.2) ³ç ð³âíîñò³ (12.2.9) âèïëèâàº, ùî<br />
∀ε >0∃Nn >N:<br />
an<br />
+ 1<br />
−ρ a n (12.2.13)<br />
ïðè n > N +1.<br />
q<br />
456 457