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n ºûªÅ¶¡»P n ºû¦V¶q

³] n ¬°¥¿¾ã¼Æ. §Ú­Ì·Q­n©w¸q nºû(¼Ú¤ó)ªÅ¶¡©M¨ä¤¤ªº¦V¶q. §Ú­Ì¥Í¬¡ªº¸gÅç¶È¤Îºû¼Æ¤£¶W¹L3ªºªÅ¶¡. ¦b°ª¤¤½Ò¥»¤¤, 2 ºû©M 3 ºûªº¦V¶q¥»¬O¥Î´X¦óªº¤èªk©w¸qªº. ·í¤Þ¤J®y¼Ð¶b«á, ¨C­Ó2ºûªÅ¶¡ªºÂI¦³¨â­Ó®y¼Ð, ¨C­Ó 2 ºû¦V¶q¦³¨â­Ó¤À¶q, ¦Ó¨C­Ó 3 ºûªÅ¶¡ªºÂI¦³¤T­Ó®y¼Ð, ¨C­Ó 3 ºû¦V¶q¦³¤T­Ó¤À¶q. ¦]¬° 2 ©Î 3 ºûªÅ¶¡ªºÂI§¹¥þ¥Ñ¨ä®y¼Ð¨M©w, ¦Ó¦V¶q¤]§¹¥þ¥Ñ¨ä¤À¶q¨M©w, ±q¥N¼ÆªºÆ[ÂI, µL½×®y¼Ð¤]¦n, ¤À¶q¤]¦n, ³£¥u¬O¹ê¼Æ¹ï©Î¤T¹ê¼Æ²Õ, ¥»½è¤W¨S¦³¤°»ò¤£¦P; ¦ý²Õ¦¨ÂIªº®y¼Ð©Î¦V¶qªº¤À¶q«á, §êºt¤£¦Pªº¨¤¦â¦Ó¤w.

¦b°ª¤¤½Ò¥»ÁÙ¦³¤@­ÓÆ[©À¥i¨Ñ§Ú­Ì§Q¥Î, ¨º«K¬O $m\times n$ ¯x°}. §Ú­Ì¥Î $\mathbb{R} $ ªí¥Ü©Ò¦³¹ê¼Æªº¶°¦X, ${\mathbb{R} }^{n}$ ªí¥Ü©Ò¦³¹ê¼Æ $1\times n$ ¯x°}ªº¶°¦X, ºÙ¤§¬° n ºûªÅ¶¡ (n-dimensional space). ${\mathbb{R} }^{n}$ ¤¤ªº¤¸¯À¤£¹L¬O n ­Ó¹ê¼Æ«ö¤@©wªº¦¸§Ç±Æ¦¨¤@¦C¦Ó¤w. ¥H¤U§Ú­Ì¤À§O±q¦V¶qªºÆ[ÂI©MÂIªºÆ[ÂI¨Ó°Q½× ${\mathbb{R} }^{n}$.

¦b°ª¤¤½Ò¥»¤¤´¿°Q½×¹L3ºû¦V¶qªººØºØ¹Bºâ. §Ú­Ì­n§â³o¨ÇÆ[©À±À¼s¨ì n ºû ¥h. ³] $x=(x_{1},x_{2},\ldots,x_{n})$ ¤Î $y=(y_{1},y_{2},\ldots,y_{n})$ ¬° ${\mathbb{R} }^{n}$ ¤¤ªº¨â­Ó¤¸¯À. $c\in {\mathbb{R} }$. ¥O

\begin{displaymath}x+y=(x_{1}+y_{1},x_{2}+y_{2},\ldots,x_{n}+y_{n}),
\end{displaymath}


\begin{displaymath}cx=(cx_{1},cx_{2},\ldots,cx_{n}),
\end{displaymath}


\begin{displaymath}(x,y)=x_{1}y_{1}+x_{2}+y_{2},\ldots,x_{n}+y_{n}).
\end{displaymath}

«h x+y, cx, (c,x) ¤À§OºÙ¬° x »P y ªº©M, x ªº c ­¿, ¤Î x »P y ªº¤º¿n (inner product). ª`·N x+y ©M cx ªº©w¸q»P¯x°}ªº¥[ªk»P­¿¼Æ©w¸q§¹¥þ¤@­P. ¦]¬°¤º¿n (x,y) ¦³®É¤]¥Î $x\cdot y$ ªí¥Ü, ¦b­^¤å¤¤¥¦¤]¥s dot product. ¤Tºû¦V¶qªÅ¶¡¤¤³o¨Ç¹Bºâ©Òº¡¨¬ªºªk«h, ¦b n ºû¦V¶qªÅ¶¡¤¤³£¤´µM¦¨¥ßŪªÌ¸Õ¦Û¦æÃÒ¤§. ·í§Ú­Ì¦b ${\mathbb{R} }^{n}$ ¤¤¦Ò¼{³o¨Ç¹Bºâ®É, ¥¦«K¥s§@ n ºû¦V¶qªÅ¶¡(n-dimensional vector space), ¨ä¤¤ªº¤¸¯À¥s§@ n ºû¦V¶q. ­Y $x=(x_{1},x_{2},\ldots,x_{n})\in [\mathbb{R} ]^{n}$, «h¹ê¼Æ $x_{1},x_{2},\ldots,x_{n}$ ¥s§@¥¦ªº¤À¶q (components). ¨C­Ó¤À¶q³£¬O 0 ªº¦V¶q¥s§@¹s¦V¶q (null vector), §Ú­Ì¤]¥Î 0 ªí¥Ü¥¦.

­Y x ¬° n ºû¦V¶q«h (x,x) ªº¥­¤è®Ú¥s§@¥¦ªºªø (magnitude,norm), ¥H $\Vert x\Vert$ ©Î |x| ªí¥Ü. ­Y¹G¦V¶q¤§ªø¬° 1, «hºÙ¸Ó¦V¶q¬°³æ¦ì (unit) ¦V¶q. ­Y $x\neq 0 $,«h $(1/{\Vert x\Vert})x$ ¬°»P x ¦P¤è¦V¤§³æ¦ì¦V¶q. ${\mathbb{R} }^{2}$ ¤¤ªº³æ¦ì¦V¶q u ³£¥i¥Hªí§@

\begin{displaymath}u=(\cos{\theta},\sin \theta)
\end{displaymath}

¤§§Î, ¦¡¤¤ $\theta$ ¬° u ©M x ¶bªº§¨¨¤. ­Y u ¬° ${\mathbb{R} }^{3}$ ¤¤ªº³æ¦ì¦V¶q, ¥O $\alpha$, $\beta$, $\gamma$ ¬° u ©M¤T®y¼Ð¶b¶¡ªº§¨¨¤, «h u ¦b¤T®yÀY¼Ð¶b¤Wªº§ë¼v¤À§O¬° $\cos \alpha$, $\cos \beta$, $\cos \gamma$. ³o¤T­Ó¼Æ¥s°µ u ªº¤è¦V¾l©¶ (directional cosines) ¦Ó u ¥i¥Hªí¦¨

\begin{displaymath}u=(\cos \alpha,\cos \beta,\cos \gamma).
\end{displaymath}

¨ä¦¸§Ú­Ì±qÂIªºÃö©À¨Ó¬Ý Rn. ¦b2ºû©Î3ºûªÅ¶¡¤¤, ÂI¤]¬O¥Î 2 ©Î 3 ­Ó¹ê¼Æ (§Y¨ä®y¼Ð) ªí¥Üªº. ±À¼s¨ì°ªºû±¡§Î, §Ú­Ì¤]§â n ºû¹ê¼Æ¦ê¬Ý¦¨ÂI. ³o¨ÇÂI©Òºc¦¨ªº¶°¦XºÙ¬° n ºûªÅ¶¡, ¤´¥Î ${\mathbb{R} }^{n}$ ªí¥Ü. ³] $x=(x_{1},x_{2},\ldots,x_{n})$ ¬°¤@ÂI. ¦pªG§â x ¬Ý¦¨ n ºû¦V¶q, «KºÙ¤§¬°¸Ó®y¼Ð¦V¶q (coordinate vector). ®y¼Ð¦V¶q¬° 0 ªºÂI¥s°µ Rn ªº­ìÂI. ³] A, B ¬°¨âÂI, ¨ä®y¼Ð¦V¶q¤À§O¬° x, y. «h y-x ¥ç¥Nªí¥H A ¬°°_ÂI, ¥H B ¬°²×ÂIªº¦V¶q, ¦³®É¥Î $\displaystyle\vec{AB}$ ¨Óªí¥Ü. A, B ¤§¶¡ªº¶ZÂ÷©w¬° $\Vert y-x\Vert$, ³q±`¥H AB ªí¥Ü. ¦b n ºûªÅ¶¡¤¤¦P®É¦Ò¼{³o¶ZÂ÷®É, ¥¦«K§@ n ºû¼Ú¤óªÅ¶¡ (Eulidean space). ÁöµM n ºûªÅ¶¡©M n ºû¼Ú¤óªÅ¶¡¦bÃö©À¤W¤£¤@¼Ë, ¦ý¬Ý¦¨¶°¦X®É¥¦­Ì³£¬O n ºûªÅ¶¡ªº¤£¦P­±¦V. §â ${\mathbb{R} }^{n}$ ¬Ý¦¨¦V¶qªÅ¶¡, «h¥¦ªº¤¸¯À x, y ¶¡«h¦³ x+y, x-y, $x\cdot y$ µ¥¹Bºâ, ¤S¦³¦V¶qªºªøªº·§©À. §â ${\mathbb{R} }^{n}$ ¬Ý¦¨¼Ú¤óªÅ¶¡, «h¥i¥H°Q½× ${\mathbb{R} }^{n}$ ¤ºÂI¶¡ªº¶ZÂ÷, ¦ýÂI©MÂI¤£¯à°µ¥[ªk, ´îªkµ¥¹Bºâ. ¦]¬°§Ú­Ì©T·N§â²Å¸¹²V¥Î, ©Ò¥H­Y x, y ¬° ${\mathbb{R} }^{n}$ ¤ºªº¨â­ÓÂI, a, b ¬°¨â­Ó¹ê¼Æ, «h ax+by ¥i¥H¥Nªí¥H ax+by ¬°®y¼Ð¦V¶qªºÂI. ¥H«á§Ú­Ì±`±`³o¼Ë¥Î. ¨Ò¦p¦b©³¤U°Q½×½u¬q©Mª½½u®É´N­n¦p¦¹¸ÑÄÀ¦b 2 ºû 3 ºûªÅ¶¡, ¤º¿n»P§¨¨¤. §ë¼v¦³±K¤ÁÃö«Y, °ª¤¤½Òµ{¤w¦³¤¶²Ð, ¨ä­nÂI¦p¤U: ³] A ©M B ¬° ${\mathbb{R} }^{n}$ ¤º¨âÂI, ¨ä®y¼Ð¦V¶q¤À§O¬° x ©M y. «h¶°¦X $\{ty+(1-t)x : 0\leq{t}\leq{1}\}$ ªí¥Ü½u¬q AB, ¦Ó¶°¦X $\{ty+(1-t)x:t\in{\mathbb{R} }\}$ ·í $A \neq{B}$ ®Éªí¥Ü³s±µ AB ªºª½½u. °²­Y A ©M B ³£¤£¬O­ìÂI O(§Y $x \neq{0}\neq{y})$, ¥O $\theta$ ªí¥Ü OA ©M OB ªº§¨¨¤, «h

\begin{displaymath}\cos\theta=\frac{(x,y)}{\Vert x\Vert\Vert y\Vert},
\end{displaymath}

¦Ó OA ¦b OB ¤Wªº§ë¼v¬°

\begin{displaymath}\Vert x\Vert\cos\theta\frac{y}{\Vert y\Vert}=\frac{(x,y)}{\Vert y\Vert^{2}}y
\end{displaymath}

¥O D ¬°¥H $\displaystyle\frac{(x,y)}{\Vert y\Vert^{2}}y$ ¬°®y¼Ð¦V¶qªºÂI. D §Y A ¦b OB ¤Wªº§ë¼v, ¬O OB ©M A ¶ZÂ÷³Ìµuªº¦a¤è.

·í $n\geq4$ ®É, ¤£¦A¯à¯u¥¿¹Ï¥Ü ${\mathbb{R} }^{n}$, ¥i¬O³o¨Ç´X¦ó«ä¸ô¨ÌµMºZ³q, ½Ð¬Ý¤U­±ªº»¡©ú: ³] A, B ¬O ${\mathbb{R} }^{n}$ ¤º¤GÂI, ¨ä®y¼Ð¦V¶q¤À§O¬° X ©M Y. ¥O

\begin{displaymath}L(t)=ty+(1-t)x, \quad {0}\leq{t}\leq{1}.
\end{displaymath}

¤U¤å¤¤§Ú­Ì±NÃÒ©ú L(t) ¬O³sµ² A, B ¤GÂIªº¦±½u¤¤ªø«×³Ìµuªº, ¦Ó¥B L(t) ªºªø­è¦n¬O A ©M B ªº¶ZÂ÷ $\Vert x-y\Vert$(¨£ $\S5$ ¨Ò 12). ÁÙ¥iÃÒ©ú ${\mathbb{R} }^{n}$ ¤¤¥ô·NÂI C ªº®y¼Ð¦V¶q¬O§_¦b L(t) ¤W, ºÝµø AC+CB=AB ¬O§_¦¨¥ß¦Ó©w(¨£²ßÃD 1). ¦]¦¹§Ú­Ì§â AB ½u¬q©w°µ ${ty+(1-t)x:0\leq{t}\leq{1}}$. ­Y $A \neq{B}$, «h A, B ©Ò¨M©wªºª½½u¦ÛµM´N©w°µ ${ty+(1-t)x:t\in{\mathbb{R} }}$ ¤F (°Ñ¬Ý²ßÃD2). ³o¼Ë©w¸q§¹¥þ²Å¦X§Ú­Ì¹ï½u¬q©Mª½½uªºª½Æ[»{ÃÑ, ¤£¥u¬O§Î¦¡¤W©M ${\mathbb{R} }^{3}$ ±¡§ÎÃþ¦ü¦Ó¤w.

¦A°²©w A ©M B ³£¤£¬O­ìÂI (§Y $x\neq{0}\neq{y}$), §ë¼vªº°ÝÃD¬Û·í©ó§ä OB ª½½u¤W©M A ³Ì±µªñªºÂI. ³] OB ½u¤W¥ô¤@ÂI¬° $\lambda{y}$, «h $\lambda{y}$ ©M A ªº¶ZÂ÷¬° $\Vert x-\lambda{y}\Vert$. ¦p¤U¥Î²³æªº¥N¼Æ­pºâ´N¥i¨M©w $\Vert x-\lambda{y}\Vert$ ªº³Ì¤p­È©Ò¦b. ¦]

\begin{eqnarray*}{\Vert x-\lambda{y}\Vert}^{2}&=&\Vert x\Vert^{2}-2(x,y)\lambda+...
...Vert x\Vert^{2}-{ \left[\frac{(x,y)}{\Vert y\Vert} \right]}^{2}.
\end{eqnarray*}


ÅãµM·í $\displaystyle\lambda=\frac{(x,y)}{\Vert y\Vert^{2}}$ ®É, $\Vert x-\lambda{y}\Vert$ ¨ú±o³Ì¤p­È. ³] D ¬°¥H $\displaystyle\frac{(x,y)}{\Vert y\Vert^{2}}y$ ¬°®y¼Ð¦V¶qªºÂI. D ºÙ¬° A ¦b OB ¤Wªº§ë¼v(projection), ¦Ó OD ºÙ¬° OA ¦b OB ¤Wªº§ë¼v; ¤]¥i¥H»¡ $\displaystyle\frac{(x,y)}{\Vert y\Vert^{2}}y$ ¬° x ¦b y ¤è¦Vªº§ë¼v. ¤S¦¹®É

\begin{displaymath}\Vert x\Vert^{2}-\frac{(x,y)^{2}}{\Vert y\Vert^{2}}={\Vert x-\lambda{y}\Vert}^{2}\geq{0}.
\end{displaymath}

¬G¦³¤£µ¥¦¡

\begin{displaymath}\displaystyle\mid(x,y)\mid\leq{\Vert x\Vert\Vert y\Vert}.
\end{displaymath}

³o«K¬O¦³¦Wªº Cauchy ¤£µ¥¦¡. ¦b³o¤£µ¥¦¡¤¤µ¥¸¹¦¨¥ßªº¥R­n±ø¥óÅãµM¬O $x=\lambda{y}$ ²{¦b¨ú $\theta\in{[0,\pi]}$, ¨Ï $\displaystyle\cos\theta=\frac{(x,y)}{\Vert x\Vert\Vert y\Vert}$ «h OA ¦b OB ¤Wªº§ë¼v¬°

\begin{displaymath}\displaystyle\frac{(x,y)}{\Vert y\Vert^{2}}y=\Vert x\Vert\cdot\cos\theta\cdot\frac{y}{\Vert y\Vert}.
\end{displaymath}

¦p¦¹ $\theta$ ¥iµø¬° OA ©M OB ªº§¨¨¤. °²¦p $\displaystyle\theta={\frac{1}{2}}\pi$, §Y(x,y)=0, «hºÙ x ©M y ««ª½ (per-pendicular) ©Î¥¿¥æ (orthogonal). ¬°¤F¤è«K°_¨£, §Ú­Ì»{¬°¹s¦V¶q©M©Ò¦³¦V¶q³£««ª½. ­Y $u\neq{0}$ ¬° ${\mathbb{R} }^{n}$ ¤¤ªº¦V¶q, «h»P u ¦P¤è¦Vªº³æ¦ì¦V¶q $u/{\Vert u\Vert}$ ªº½Ñ¤À¦V¶q«K¬O u ©M½Ñ®y¼Ð¶b¶¡§¨¨¤ªº¾l©¶, §Ú­Ì¤´ºÙ¤§¬° u ªº¤è¦V¾l©¶. ¥H¤W§Ú­Ì¦b ${\mathbb{R} }^{n}$ ¤¤¤Þ¶i½u¬q¡Bª½½u¡B§¨¨¤¡B§ë¼vµ¥Æ[©À, ¤£¥u¾A¦X­ì¨Ó ${\mathbb{R} }^{3}$ ªº¤½¦¡, ¦Ó¥B©M ${\mathbb{R} }^{3}$ ¤¤ªºª½Æ[§¹¥þ¬Û®e. Cauchy ¤£µ¥¦¡¬O¤W­±­pºâ¤¤³Ì­«­nªºµ²ªG, §Ú­Ì¯S¦a±N¥¦¥Î©w²zªº§Î¦¡ªí¥X:
\begin{theorem}$($\emph{Cauchy ineqiality}$)$ . ³] $x,y\in{{\mathbb{R} }^{n}}$ ,...
...y=0$\space ©Î¦³ $\lambda\in{\mathbb{R} }$\space ¨Ï $x=\lambda y$ .
\end{theorem}

¤U­±ªº©w²z¬O¥j¨åªº²¦¤ó©w²z (Pythagoras theorem) ªº±À¼s, ½ÐŪªÌ¦Û¤vÃÒ©ú.


\begin{theorem}If the two vectors x and y in ${\mathbb{R} }^n$\space are orthogo...
...{\Vert x+y\Vert}^2=\Vert x\Vert^2+\Vert y\Vert^2.
\end{displaymath}\end{theorem}

¥H¤U´X­Óµ²ªG³£¥i±q Cauchy ¤£µ¥¦¡±o¨ì:
\begin{theorem}$($\emph{¤T¨¤§Î¤£µ¥¦¡}$)$ . ³] $x,y\in R^n$ , «h
\begin{displayma...
...±ø¥ó¬O $x=0$ , ©Î¦³¤@¹ê¼Æ $\lambda\geq{0}$ ,
¨Ï±o $y=\lambda{x}$ .
\end{theorem}

ÃÒ©ú. ±q Cauchy ¤£µ¥¦¡±o

\begin{eqnarray*}{\Vert x+y\Vert}^2&=&(x+y,x+y)=\Vert x\Vert^2+2(x,y)+\Vert y\Ve...
...ot{\Vert y\Vert}+\Vert y\Vert^2}={(\Vert x\Vert+\Vert y\Vert)}^2
\end{eqnarray*}


¨âÃä¶}¤è, «K¥i±o¨ì©Ò¨Dªº¤£µ¥¦¡.

­Yµ¥¸¹¦¨¤§, «h $(x,y)=\Vert x\Vert\Vert y\Vert$, ±q¦Ó©Î x=0, ©Î¦³¤@¥¿¹ê¼Æ $\lambda$ ¦b, ¨Ï $y=\lambda x$. ¤Ï¤§­Y¦¹±ø¥ó¦¨¥ß, «h©öÃÒ¤£µ¥¦¡¤¤µ¥¸¹¦¨¥ß.

³o¤£µ¥¦¡«K¥s°µ¤T¨¤§Î¤£µ¥¦¡ (triangle inequality). §Q¥Î¥¦, §Ú­Ì¥ß§Y±o¨ì
\begin{theorem}³] $A$ , $B$ , $C$\space ¬° $\mathbb{R} ^n$\space ªº¤TÂI. «h\\
¥...
... , $B$ , $C$\space ¤TÂI¦@½u,
¥B $B$\space ¦b $A$ , $C$\space ¤§¶¡.
\end{theorem}

¦b¥»©w²z¤¤©Ò¦CÁ|ªº¶ZÂ÷ªº¤T­Ó©Ê½è¤¤, ¤þ¤]¥s°µ¤T¨¤§Î¤£µ¥¦¡. ³o¼Ë±o¨ìªº¶ZÂ÷©M§Ú­Ìªºª½Æ[«D±`±µªñ. ¦]¦¹§Ú­Ì»{¬° ${\mathbb{R} }^{n}$ ¤¤¶ZÂ÷ªºÆ[©À¬O 2 ¤Î 3 ºûªÅ¶¡¤¤¶ZÂ÷Æ[©Àªº¦ÛµM±À¼s.


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1999-06-27