±`·L¤À¤èµ{¡G¯Å¼Æ¸Ñ
¦³®É ODE ªº¸Ñ¤£«ê¦n¬O¤Ö¼Æ±`¨£¨ç¼Æªº²Õ¦X¡A¥¦Ì¦³³z¹L¯Å¼Æªí¥X ªº¾÷·|¡C¤F¸Ñ¨D¦¹ ¯Å¼Æ¸Ñ¦p¦ó¥i¥H³Q¨D±o¡A«h¬O¥»¸`ªº¾Ç²ß¥Ø¼Ð¡C
¹w³Æª¾ÃÑ¡]·L¿n¤À±Ð¬ì®Ñ¡^¡G¼Æ¦C»P¯Å¼Æ¡B¯Å¼ÆªºÀÄ´²©Ê¡]¦¬Àĵo´²©Ê¡^´ú¸Õ¡C
¦^ÅU¡G¦ó¿×¯Å¼Æ¡H·|¦¬ÀĪº»P¤£¦¬ÀĪº¯Å¼Æ¦³¦ó¤£¦P¡H
¥H y'' + y = 0 ¬°¨Ò¡G
°²³]¸Ñ¥iªí¬° y = a0 + a1 x + a2 x 2 + a3 x 3 + ...
¦p¦ó¨D¥X a0¡Ba1¡B a3 ...
¦]¬°¦³µL¦hÓ¡An¦³³W«hªº¤~¯à¸Ñ±o¥X
ÃöÁä 1¡G«Ý©w¤§¸Ñ«Y¼Æ¦³¤@¨t¦C©T©wªºÃö«Y¡]±N¸Ñªº¯Å¼Æ§Î¦¡¥N¤Jì¤èµ{¦¡·|º¡¨¬¡^
ÃöÁä 2¡G¹ï¸Ñ y(x) ¦Ó¨¥¡Ax ¤£ºÞÅܰʬҬO¸Ñ¡A´£¨Ñ¤F¨¬°÷¦hªº±ø¥ó¦¡¥H©w¥X a0¡Ba1¡B a3 ...
½Ò¥»¸Ô²Ó¦a¸Ñ¥X¡]¨£¤§ p.85¡^
y = a0 ( 1 - x2/2! + x4/4! - x6/6! + ... ) + a1 (x/1! - x3/3! + x5/5! - ... )
¨Æ¹ê¤W¡A¨âÓ¨í¸¹ùرªºªF¦è´N¬O¥¿©¶»P¾l©¶¨ç¼Æªº®õ°Ç®i¶}
¡]¦ó¿× ®õ°Ç®i¶}¦¡¡H¤W¦¸Á¿¹L¤F¡C¥t¨£·L¿n¤À½Ò¥»©Î ºû°ò¦Ê¬ì¡A«n¡C¡^
¤G¶¥¡]¥¼ª¾¨ç¼Æ·L¤À¨â¦¸¡^½u©Ê¡]¤@¦¸¤è¡^ODE ªº¯Å¼Æ¸Ñ
¤G¶¥½u©Ê ODE ¤@¯ë©Ê§Î¦¡¬°
y'' + P(x) y' + Q(x) y = 0
«ä¦Ò¡G¤W¦¡n¦³·Qn¦³¯Å¼Æ¸Ñ¡AP(x)¡BQ(x) n²Å¦X¤°»ò¯S©Ê¡H
¡]´£¥Ü¡Gªí¦¨¯Å¼Æ¡^
Frobenious and Fuchs ©w²z
P(x) ¡BQ(x) ¦b x = α ³B¯à§_¼g¥X¥i¦¬ÀĪº®õ°Ç®i¶}¡A¨M©w¤F¦³µL¯Å¼Æ¸Ñ¡]½Ò¥»¤£¤¶²ÐÃÒ©ú¡^
¯à®õ°Ç®i¶} regular point
¤£¯à®õ°Ç®i¶} signular point
P(x) = λ(x) / ( x-α) ¥B Q(x) = μ(x) / ( x-α)2¡A¨ºÂI¥s "regular" sigular point
¤À¤TºØ±¡ªp
(1) P(x)¡B Q(x) ³£¬O regular
ODE ¦³¨âÓ¬Û²§¸Ñ¡A
y(x) = Σn=1∞ aλ (x - α)λ
(2) sigular point is regular (§Y λ(x) ¡Bμ(x) regular)
y(x) = Σn=1∞ aλ (x - α) λ+ ρ
¨ä¤¤ ρ ¬°¬Y±`¼Æ
(3) λ(x) ¡B μ(x) singular
Ex 2.14
§ä 4x y'' + 2y ' + y = 0 ªº³q¸Ñ
§â³Ì§C¦¸ x ¦¸¤è«Y¼Æ¾ã²z¥X¡A¨Ã¥BÅé»{ a0 ¤£¬°¹s¡A´N¦³ indicial equation (http://mathworld.wolfram.com/IndicialEquation.html)
4ρ (ρ -1) + 2 ρ = 0
§Y 2 ρ ( 2 ρ - 1 ) = 0 ¡A¦p¦¹±o¨âÓ®Ú ρ = 0 ¤Î ρ = 1/2
¤W¨ÒÄÝ indicial equation ¦³¨â¬Û²§®Ú¤§±¡§Î¡A ¤@Ó ODE ¤´¥t¦³ (a) «®Ú¥H¤Î (b) ¨â®Ú®t¤@¾ã¼Æªºª¬ªp¡C
¹ê¨Ò±À¾É
x2 y'' + x g(x) y' + h(x) y = 0
¨ä¤¤ g(x) »P h(x) ¦b x=0 ³BµL¦¸¥i·L (¥s analytic)
ª`·N¤W¦¡¼g¦¨ y'' + P(x)y' + Q(x)y = 0 ¤DÄÝ reqular sigular point Ãþ§O¡A¬G¨ä³q¸Ñ¦³
y(x) = xr Σm=0∞ am xm
Case 1¡]¬Û²§®Ú¡B¤£®t¾ã¼Æ¡^
¨âӸѽu©Ê¿W¥ß¡A¦] y1/y2 «D±`¼Æ
Case 2¡]«®Ú¡^
¥ý±o y1(x)
¦A¥O y2(x) = u(x) y1(x) ¡A¥N¦^ì ODE ¡A§Q¥Îì y1(x) ¤w¬O¸Ñªº±ø¥ó¡A¼g¤U u'' ¤Î u' ©Ò¶·º¡¨¬ªº¤èµ{¦¡¡A¦p¦¹²Ê²¤¦a¨D±o u(x) À³¦³ªº§Î¦¡¡]§C¶¥¶µ¨ãÅé±o¨ì¡^¦ÓÀò±o±o u(x)y1(x) ªº ¨ãÅé§Î¦¡¡C
Case 3 ¡]¬Û²§®Ú¡B®t¤@¾ã¼Æ¡^
Ãþ¦ü«®Ú±¡§Î¡A¥ý±o y1(x)
¡]¾Ç¼Æ¾Ç¡A¬O±À¾É¤ñ¸û«n¡HÁÙ¬O³Ð·N¤ñ¸û«n¡H¡^
Ex 2.15 ¡]indicial equation ¨â®Ú®t¤@Ó¾ã¼Æªº¨Ò¤l¡^
x2 y'' + x y' + (x2 - 1/4) y = 0