2020-2ํ•™๊ธฐ, ๋Œ€ํ•™์—์„œ โ€˜์‘์šฉ๋ณต์†Œํ•จ์ˆ˜๋ก โ€™ ์ˆ˜์—…์„ ๋“ฃ๊ณ  ๊ณต๋ถ€ํ•œ ๋ฐ”๋ฅผ ์ •๋ฆฌํ•œ ๊ธ€์ž…๋‹ˆ๋‹ค. ์ง€์ ์€ ์–ธ์ œ๋‚˜ ํ™˜์˜์ž…๋‹ˆ๋‹ค :)

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2020-2ํ•™๊ธฐ, ๋Œ€ํ•™์—์„œ โ€˜์‘์šฉ๋ณต์†Œํ•จ์ˆ˜๋ก โ€™ ์ˆ˜์—…์„ ๋“ฃ๊ณ  ๊ณต๋ถ€ํ•œ ๋ฐ”๋ฅผ ์ •๋ฆฌํ•œ ๊ธ€์ž…๋‹ˆ๋‹ค. ์ง€์ ์€ ์–ธ์ œ๋‚˜ ํ™˜์˜์ž…๋‹ˆ๋‹ค :)


์ด์ „๊นŒ์ง€๋Š” residue๋ฅผ ๊ตฌํ•˜๊ธฐ ์œ„ํ•ด ํ•จ์ˆ˜ f(z)์˜ ๋กœ๋ž‘ ๊ธ‰์ˆ˜๋ฅผ ๊ตฌํ•ด b1์„ ๊ตฌํ•˜๋Š” ๋ฐฉ๋ฒ•์„ ์‚ฌ์šฉํ–ˆ๋‹ค.

๊ทธ๋Ÿฐ๋ฐ ๋งŒ์•ฝ f(z)๊ฐ€ niceํ•œ ์กฐ๊ฑด์„ ๊ฐ€์ง€๊ณ  ์žˆ์Œ์ด ํ™•์ธ๋œ๋‹ค๋ฉด, ๋กœ๋ž‘ ๊ธ‰์ˆ˜๋ฅผ ๊ตฌํ•˜์ง€ ์•Š๊ณ ๋„ ์†์‰ฝ๊ฒŒ residue๋ฅผ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค!!


review. Cauchyโ€™s residue theorem

โˆฎCf(z)dz=2ฯ€i(โˆ‘k=1nResz=zkf(z))


review.
Let p(z) and q(z) be analytic functions. Supp. that q(z) has a zero of m-th order at z=z0 and p(z0)โ‰ 0.

THEN

f(z)=p(z)q(z)

has a pole at z=z0 of order m


Formulas for residuesPermalink

simple poles at z0Permalink

Consider

f(z)=p(z)q(z)

where p(z) and q(z) are analytic functions. Supp. that q(z) has a simple zero at z=z0 and p(z0)โ‰ 0.

THEN

Resz=z0f(z)=limzโ†’z0(zโˆ’z0)โ‹…f(z)=p(z0)qโ€ฒ(z0)

proof.
We can express f(z) as follows

f(z)=b1(zโˆ’z0)+a1+a2(zโˆ’z0)+โ‹ฏ

THEN

(zโˆ’z0)f(z)=b1+a1(zโˆ’z0)+a2(zโˆ’z0)2+โ‹ฏ

THEN, take limit for that

limzโ†’z0(zโˆ’z0)f(z)=b1

๋”ฐ๋ผ์„œ limzโ†’z0(zโˆ’z0)f(z)๋ฅผ ํ†ตํ•ด residue๋ฅผ ์†์‰ฝ๊ฒŒ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค!!


๊ทธ ๋‹ค์Œ ๊ณต์‹์€ limzโ†’z0(zโˆ’z0)f(z)์— ์•ฝ๊ฐ„์˜ trick์„ ์จ์„œ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค.

limzโ†’z0(zโˆ’z0)f(z)=limzโ†’z0(zโˆ’z0)p(z)q(z)=limzโ†’z0p(z)q(z)โˆ’q(z0)zโˆ’z0=p(z0)qโ€ฒ(z0)

poles of order mPermalink

Resz=z0f(z)=1(mโˆ’1)!limzโ†’z0(ddz)(mโˆ’1)[(zโˆ’z0)mf(z)]

proof.

f(z)=bm(zโˆ’z0)m+โ‹ฏ+b1(zโˆ’z0)+a0+a1(zโˆ’z0)+โ‹ฏ(zโˆ’z0)mf(z)=bm+โ‹ฏ+b1(zโˆ’z0)m+a0(zโˆ’z0)m+โ‹ฏ

(zโˆ’z0)mf(z)์—์„œ b1์„ ์–ป๊ธฐ ์œ„ํ•ด์„  (mโˆ’1)๋ฒˆ ๋ฏธ๋ถ„ํ•ด์•ผ ํ•œ๋‹ค.

(ddz)(mโˆ’1)[(zโˆ’z0)mf(z)]=b1(mโˆ’1)!+a0(mโˆ’1)!(zโˆ’z0)+โ‹ฏlimzโ†’z0(ddz)(mโˆ’1)[(zโˆ’z0)mf(z)]=b1(mโˆ’1)!

๋”ฐ๋ผ์„œ order m์— ๋Œ€ํ•œ b1=Resz=z0f(z)๋Š” ์œ„์™€ ๊ฐ™์€ ์‹์œผ๋กœ ๊ตฌํ•  ์ˆ˜ ์žˆ๋‹ค!


Example.

f(z)=9z+iz3+z

Find residues of f(z).


Sol.
q(z)=z3+z=z(z2+1)

๋”ฐ๋ผ์„œ f(z)์˜ pole์€

z=0,+i,โˆ’i

์ด๊ณ , ๋ชจ๋‘ simple pole์ด๋‹ค.

(i) residue for z=0

zf(z)=zโ‹…9z+iz(z2+1)=9z+iz2+1limzโ†’0zf(z)=i1=i

๋”ฐ๋ผ์„œ Resz=0f(z)=i.

(ii), (iii) residue for z=+i,โˆ’i๋Š” ์ƒ-๋žต