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์ˆ˜ํ•™๊ณผ ๋ณต์ˆ˜์ „๊ณต์„ ์œ„ํ•ด ์กธ์—… ๋งˆ์ง€๋ง‰ ํ•™๊ธฐ์— โ€œ์ˆ˜์น˜ํ•ด์„๊ฐœ๋ก โ€ ์ˆ˜์—…์„ ๋“ฃ๊ฒŒ ๋˜์—ˆ์Šต๋‹ˆ๋‹ค. ์ˆ˜ํ•™๊ณผ ์กธ์—…์‹œํ—˜๋„ ๊ฒธ์‚ฌ๊ฒธ์‚ฌ ์ค€๋น„ํ•  ๊ฒธ ํ™”์ดํŒ… ํ•ด๋ด…์‹œ๋‹ค!! ์ „์ฒด ํฌ์ŠคํŠธ๋Š” โ€œNumerical Analysisโ€œ์—์„œ ํ™•์ธํ•  ์ˆ˜ ์žˆ์Šต๋‹ˆ๋‹ค.

๋น„์„ ํ˜• ๋ฐฉ์ •์‹ $f(x) = 0$์˜ ๊ทผ์„ ์ฐพ๊ธฐ ์œ„ํ•œ ์ˆ˜์น˜์  ๋ฐฉ๋ฒ• ์ž…๋‹ˆ๋‹ค.

Lipchitz

Locally Convergence

FPI์˜ ๊ฒฝ์šฐ๋Š” ์ˆ˜๋ ด์ด ๋ฐ˜๋“œ์‹œ ๋ณด์žฅ๋˜์ง€ ์•Š๊ณ , ์ดˆ๊ธฐ๊ฐ’์— ์˜์กดํ•  ์ˆ˜ ์žˆ์Šต๋‹ˆ๋‹ค.

๊ตฌ๊ฐ„ $[a, b]$์—์„œ $| gโ€™(x) | < 1$์„ ๋งŒ์กฑํ•œ๋‹ค๋ฉด, ์ด ๊ตฌ๊ฐ„์—์„œ ์ˆ˜๋ ด์„ ๊ธฐ๋Œ€ํ•  ์ˆ˜ ์žˆ๋Š” ํ›„๋ณด ์˜์—ญ์ž…๋‹ˆ๋‹ค. ํ•˜์ง€๋งŒ, ์ด ๊ตฌ๊ฐ„ ๋‚ด์˜ ๋ชจ๋“  ์ ์—์„œ ์ˆ˜๋ ด์ด ๋ณด์žฅ๋œ๋‹ค๊ณ  ๋งํ•  ์ˆ˜๋Š” ์—†์Šต๋‹ˆ๋‹ค.

์ค‘์š”ํ•œ ๊ฒƒ์€

  • ์ดˆ๊ธฐ๊ฐ’์ด ์ˆ˜๋ ด ์˜์—ญ $[a, b]$ ๋‚ด์— ์กด์žฌํ•ด์•ผ ํ•˜๊ณ ,
  • FPI์˜ ๋ฐ˜๋ณต์„ ํ†ตํ•ด ์ˆ˜๋ ด ์„ฑ์งˆ์ด ์œ ์ง€ ๋˜์–ด์•ผ ํ•ฉ๋‹ˆ๋‹ค.

์ฆ‰, $[a, b]$ ์ˆ˜๋ ด๊ฐ’์— ๋Œ€ํ•œ ์ค‘์š”ํ•œ ํžŒํŠธ๋ฅผ ์ค„ ๋ฟ ์ž…๋‹ˆ๋‹ค.

๊ทธ๋ž˜์„œ ์ˆ˜๋ ด์— ๋Œ€ํ•œ ์กฐ๊ฑด์œผ๋กœ ์ดˆ๊ธฐ๊ฐ’ $p_0$๊ฐ€ ๊ณ ์ •์  $r$์— ์ถฉ๋ถ„ํžˆ ๊ฐ€๊นŒ์›Œ์•ผ ์„ ํ˜• ์ˆ˜๋ ด์ด ๋ณด์žฅ ๋ฉ๋‹ˆ๋‹ค.

Fixed-Point Theorem

Let $g \in C[a, b]$ be s.t. $g(x) \in [a, b]$ for all $x$ in $[a, b]$.

Supp. that $gโ€™$ exists on $(a, b)$ and there is a constant $0 < K < 1$ s.t. for all $x \in (a, b)$,

\[\| g'(x) \| \le K\]

Then for any number $p_0$ in $[a, b]$, the sequence defined by $p_n = g(p_{n-1})$ for $n \ge 1$, converges to the unique fixed point $r$ in $[a, b]$.

์•„๊นŒ๋Š” FPI์ด ๊ตญ์†Œ ์ˆ˜๋ ด ํ•œ๋‹ค๊ณ  ํ•˜์˜€๋Š”๋ฐ, ์ด ์ •๋ฆฌ์—์„œ๋Š” ์ „์—ญ์ ์œผ๋กœ ์œ ์ผํ•˜๊ฒŒ ์ˆ˜๋ ด ํ•œ๋‹ค๊ณ  ํ•ฉ๋‹ˆ๋‹ค. ๋‘˜์ด ๋ญ๊ฐ€ ๋‹ค๋ฅธ ๊ฑธ๊นŒ์š”??

๋‘˜์˜ ์ฐจ์ด์ ์€ ํ•จ์ˆ˜ $g(x)$๊ฐ€ ์ž๊ธฐ ์‚ฌ์ƒ์„ ๋งŒ์กฑํ•˜๋Š”์ง€ ์ž…๋‹ˆ๋‹ค. ๋‘˜๋‹ค ์ˆ˜์ถ• ์‚ฌ์ƒ์ธ $| gโ€™(x) | < 1$์— ๋Œ€ํ•ด์„œ๋Š” ๋งŒ์กฑ์„ ํ•˜์ง€๋งŒ, ์ „์—ญ ์œ ์ผ ์ˆ˜๋ ด์„ ์œ„ํ•ด์„œ๋Š” ์ž๊ธฐ ์‚ฌ์ƒ์ด ๋งŒ์กฑ ๋˜์–ด์•ผ ํ•ฉ๋‹ˆ๋‹ค.

$r$ ๊ทผ์ฒ˜์—์„œ ์ˆ˜์ถ• ์‚ฌ์ƒ๋งŒ์„ ๋งŒ์กฑํ•œ๋‹ค๋ฉด, ๊ทธ๊ฒƒ์€ $r$ ๊ทผ์ฒ˜์—์„œ๋งŒ FPI์— ์˜ํ•ด ์ˆ˜๋ ดํ•œ๋‹ค๋Š” ๊ฒƒ์„ ๋ณด์žฅํ•˜๊ธฐ ๋•Œ๋ฌธ์— ๊ตญ์†Œ ์ˆ˜๋ ด์ด ๋ฉ๋‹ˆ๋‹ค.

Error Bound

Fixed-point Iteration์ด ์œ„์˜ ๊ณ ์ •์  ์ •๋ฆฌ๋ฅผ ๋งŒ์กฑํ•˜๋Š” ์ƒํ™ฉ์ด๋ผ๋ฉด, ์ด ๊ทผ์‚ฌ๋ฒ•์— ๋Œ€ํ•œ ์˜ค์ฐจ ๊ฒฝ๊ณ„๋ฅผ ์œ ๋„ํ•  ์ˆ˜ ์žˆ์Šต๋‹ˆ๋‹ค. ์ด๋•Œ, 2๊ฐ€์ง€ ์˜ค์ฐจ ๊ฒฝ๊ณ„๋ฅผ ์œ ๋„ํ•  ์ˆ˜ ์žˆ์Šต๋‹ˆ๋‹ค.

Error Bound - 1

If $g$ satisfies the โ€œFixed-point Theoremโ€, then bounds for the error involved in suing $p_n$ to approximate the fixed point $r$ are given by

\[\| p_n - r \| \le K^n \max \left\{ p_0 - a, b - p_0 \right\}\]

TODO: ์ฆ๋ช…

Error Bound - 2

If $g$ satisfies the โ€œFixed-point Theoremโ€, then bounds for the error involved in suing $p_n$ to approximate the fixed point $r$ are given by

\[\| p_n - r \| \le \frac{K^n}{1-K} \| p_1 - p_0 \|\]

TODO: ์ฆ๋ช…

๋‘˜ ์ค‘ ์–ด๋–ค ๊ฒƒ์„ ์“ฐ๋Š”๊ฒŒ ์ข‹์„๊นŒ?

TODO