#61
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¡çÁÕ¤èÒà»ç¹Êͧà·èҢͧ͹ءÃÁÍѹ·ÕèáÅéÇ䧤ÃѺ
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#62
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ÁÒä´éÍÂèÒ§ääÃѺ ?
¢Í Hint ˹èͤÃѺ
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#63
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¡çÅͧàÍÒ 2 ¤Ù³Í¹Ø¡ÃÁÍѹ·ÕèáÅéÇ´ÙÊÔ¤ÃѺ
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#64
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¢Íº¤Ø³¤ÃѺ (¹Ö¡àÃ×èͧ§èÒÂæá¤è¹ÕéäÁèÍÍ¡ )
we know that $$\sum_{n=1}^{\infty}\frac{\sin n}{n}$$ converges to $\frac{\pi-1}{2}$. and now I want to know series $$\sum_{n=1}^{\infty}\frac{\cos n}{n}$$ converges to ...? O.K. now I know that $$\displaystyle{\sum_{n=1}^{\infty}\frac{\cos n}{n}=-\frac{\ln(2-2\cos1)}{2}}$$ but,I don't know how to proof.
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âÅ¡¹ÕéÁÕ¤¹ÍÂÙè 10 »ÃÐàÀ· ¤×Í ¤¹·Õèà¢éÒã¨àÅ¢°Ò¹Êͧ áÅФ¹·ÕèäÁèà¢éÒ㨠26 ÁÕ¹Ò¤Á 2007 00:53 : ¢éͤÇÒÁ¹Õé¶Ù¡á¡éä¢áÅéÇ 1 ¤ÃÑé§, ¤ÃÑé§ÅèÒÊØ´â´Â¤Ø³ nongtum à˵ؼÅ: triple posts merged |
#65
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This is probably could be done with the "$C+iS$" method, where $$ C= \cos \theta + \frac{\cos 2\theta}{2} + \frac{\cos 3\theta}{3} + \cdots $$
Hope someone will show us how to prove it rigorously. |
#66
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prove
$$\cos x=\frac{e^{ix}+e^{-ix}}{2} $$ $$ \sum \frac{\cos n}{n}=\frac12(\sum \frac{e^{in}}{n}+ \sum \frac{e^{-in}}{n}) $$ $$ =\frac12\bigg(-\ln(1-e^i)-\ln(1-e^{-i})\bigg)=-\frac{\ln(2-2\cos1)}{2} $$ $$\displaystyle{\sum_{n=1}^{\infty}\frac{\cos n}{n}=-\frac{\ln(2-2\cos1)}{2}}$$
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âÅ¡¹ÕéÁÕ¤¹ÍÂÙè 10 »ÃÐàÀ· ¤×Í ¤¹·Õèà¢éÒã¨àÅ¢°Ò¹Êͧ áÅФ¹·ÕèäÁèà¢éÒ㨠09 µØÅÒ¤Á 2006 13:40 : ¢éͤÇÒÁ¹Õé¶Ù¡á¡éä¢áÅéÇ 1 ¤ÃÑé§, ¤ÃÑé§ÅèÒÊØ´â´Â¤Ø³ Mastermander |
#67
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Why is $$ \sum_{n=1}^\infty \frac{e^{in}}{n} = -\ln(1-e^i) \, ? $$
10 µØÅÒ¤Á 2006 10:46 : ¢éͤÇÒÁ¹Õé¶Ù¡á¡éä¢áÅéÇ 1 ¤ÃÑé§, ¤ÃÑé§ÅèÒÊØ´â´Â¤Ø³ warut |
#68
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¤Ø³ warut ÁÕÍÐäÃáͺὧÃÖà»ÅèÒ¤ÃѺ
$$\sum_{n=1}^{\infty} \frac{x^n}{n}=\sum_{n=1}^{\infty} \int_0^x t^{n-1} dt$$ $$=\int_0^x \sum_{n=1}^{\infty} t^{n-1} \ dt $$ $$=\int_0^x \frac{dt}{1-t}=-\ln(1-x)$$
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âÅ¡¹ÕéÁÕ¤¹ÍÂÙè 10 »ÃÐàÀ· ¤×Í ¤¹·Õèà¢éÒã¨àÅ¢°Ò¹Êͧ áÅФ¹·ÕèäÁèà¢éÒ㨠|
#69
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¶éÒ·ÕèÁҢͧÊÙµÃà»ç¹ÍÂèÒ§·ÕèÇèÒ $x$ ¡çµéͧà»ç¹¨Ó¹Ç¹¨ÃÔ§¤ÃѺ »ÑËÒÁѹÍÂÙè·ÕèÇÔ¸Õ¾ÔÊÙ¨¹ìÁѹäÁè rigorous ¹èФÃѺ ¶Ö§áÁéÇèÒ¨Ðä´é¤ÓµÍºÍÍ¡ÁÒ¶Ù¡µéͧ¡çµÒÁ
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#70
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µéͧ·ÓÍÂèÒ§ääÃѺ
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#71
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¡ç¹èÒ¹¹ÐÊÔ¤ÃѺ ¼Á¤Ô´ÇèÒ¶éҨзӡ礧µéͧ¡ÃШÒ Maclaurin's series ¢Í§ $ \ln (1-z) $ áÅéÇáÊ´§ÇèÒ͹ءÃÁ¹Õéãªéä´é·Õè¨Ø´ $ z= e^{i\theta} \ne 1 $ ´éÇ â´Âãªé Abel's Theorem áÅФÇÒÁ¨ÃÔ§·ÕèÇèÒ͹ءÃÁ $$ \sum_{n=1}^\infty \frac{e^{in\theta}}{n} $$ ÅÙèà¢éÒàÁ×èÍ $ \theta \ne 0 $ «Ö觹èҨоÔÊÙ¨¹ìä´éâ´Âá¡ÍÍ¡à»ç¹ real & imaginary parts áÅéÇãªé Dirichlet's Test ÁÑ駤ÃѺ
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#72
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¡ÒáÃШÒ $\ln (1-z)$ Laurent's Series ¨ÐªèÇÂä´éäËÁ¤ÃѺ ??
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#73
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Áѹ¡çä´é͹ءÃÁà´ÕÂǡѹáËÅФÃѺ à¾ÃÒзÕè¨Ø´ $z=0$ äÁèä´éà»ç¹ singularity
»ÑËҢͧ¡ÒÃÅÙèà¢éÒÁѹÍÂÙè·Õè¢Íº¢Í§ circle of convergence $|z|<1$ ¹èФÃѺ |
#74
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ºÑ§àÍÔä»à¨ÍÁÒ¤ÃѺ
$$ \sum_{n=1}^{\infty} \frac{\sin nx}{n} = \frac{\pi-x}{2} \quad ,0 < x < 2\pi $$ $$ \sum_{n=1}^{\infty} \frac{\cos nx}{n} = -\ln \mid 2\sin\frac{x}{2}\mid \quad ,0 < x < 2\pi $$ ·ÕèÁÒ ÁÒ¨Ò¡ Fourier expansion ¢Í§¿Ñ§¡ìªÑ¹·Ò§¢ÇÒÁ×ͤÃѺ áÅÐÃÙéÊÖ¡ÇèÒ ¨Ò¡ Fourier series ÍѹÅèÒ§ ¨Ðä´é ÍÔ¹·Ôà¡ÃµÊÇÂæ ºÒ§ÍÂèÒ§µÔ´ÁÒ´éÇ ¹Ñ蹤×Í $$ \int_0^{\pi} \ln(\sin x)\cos(2nx) \,\,dx = \frac{-\pi}{2n} $$ «Ö觼ÁÂѧäÁèä´éÅͧÍÔ¹·Ôà¡Ãµ´éÇÂÁ×͹ФÃѺ á¤è check¨Ò¡¤ÍÁáÅéǾºÇèÒà»ç¹µÑǹÕé
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#75
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¢Íº¤Ø³¤ÃѺ Fourier series Íѹº¹¹Ñè¹¼ÁÃÙé¨Ñ¡ áµè¢Í§ÍѹÅèÒ§¹Õè¿Ñ§¡ìªÑ¹·Ò§´éÒ¹¢ÇÒÁѹ unbounded (Êѧࡵ·Õè¨Ø´ $x=0$) áÅéÇÁѹ¨ÐËÒ Fourier series ä´éàËÃͤÃѺ ¼Áà¤ÂËÒáµè Fourier series ¢Í§¿Ñ§¡ìªÑ¹§èÒÂæ áÅéÇẺ¹Õé¨Ð¶×ÍÇèÒà»ç¹ piecewise continuous ÃÖà»ÅèÒ ã¤Ã·ÃÒºªèǺ͡·Õ¤ÃѺ
¶éÒËÒ¡ÇèÒÁѹà»ç¹ Fourier series ¨ÃÔ§æ àÃÒ¨Ðä´éÇèҤӵͺÍѹ˹Öè§ÊÓËÃѺ¤Ó¶ÒÁ¢Í§¤Ø³ Mastermander ¤×Í¡ÒáÃШÒ Fourier series ¢Í§¿Ñ§¡ìªÑ¹´Ñ§¡ÅèÒÇ áµè¡çÂѧµéͧ integrate µÑǹÑé¹ãËéä´é´éǹФÃѺ |
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