2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)

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2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)


μ•žμ—μ„œ 닀룬 Gaussian Integer와 Multiplicative Normμ—μ„œμ˜ ν˜„μƒμ„ 잘 μ΄μš©ν•˜λ©΄, β€œνŽ˜λ₯΄λ§ˆμ˜ 두 제곱수 정리”λ₯Ό 증λͺ…ν•  수 μžˆλ‹€!!




Theorem 47.10 Fermat’s p=a2+b2 Theorem

Let p≠2 be a prime in Z.

Then p=a2+b2 for a,b∈Z

⟺ p≑1 (mod 4).


proof.

(⟹)

First, Supp. that p=a2+b2.

Now a and b cannot both be even or both be odd since p is an odd number.

If a=2r and b=2s+1,

then a2+b2=4r2+4(s2+s)+1.

So p≑1 (mod 4).

(⟸)

Supp. that p≑1 (mod 4).

finite field Zp의 multiplicative group을 μƒκ°ν•΄λ³΄μž.

그러면, (Zpβˆ—,Γ—)λŠ” order pβˆ’1의 cyclic group이닀.

μ΄λ•Œ p≑1 (mod 4)에 μ˜ν•΄ 4λŠ” pβˆ’1의 divisorμž„μ΄ μœ λ„λœλ‹€.

λ”°λΌμ„œ Zpβˆ—λŠ” multiplicative orderκ°€ 4인 μ›μ†Œ n을 ν¬ν•¨ν•œλ‹€. (cyclic group의 경우 Lagrange thm의 역이 성립)

그리고 n2λŠ” multiplicative order 2λ₯Ό κ°€μ§„λ‹€.
(κ°„λ‹¨ν•˜κ²Œ μƒκ°ν•˜λ©΄, n이 λ›°λŠ” μŠ€ν…μ„ 절반만 λ›°κΈ° λ•Œλ¬Έμ— 4/2=2의 μœ„μˆ˜λ₯Ό κ°–λŠ” 것)
(addidtive groupμ—μ„œ nκ³Ό 2n의 μœ„μˆ˜μ™€ μƒν†΅ν•˜λŠ” λΆ€λΆ„)

λ”°λΌμ„œ n2의 μœ„μˆ˜κ°€ 2μ΄λ―€λ‘œ Zpβˆ—μ—μ„œ n2=βˆ’1이 λœλ‹€.

λ”°λΌμ„œ Z에선 n2β‰‘βˆ’1 (mod p)κ°€ λœλ‹€.

즉, Zμ—μ„œ p∣(n2+1)κ°€ λœλ‹€.


p와 n2+1의 관계λ₯Ό μ΄λ²ˆμ—” Z[i]μ—μ„œ 바라보면 μ•„λž˜μ™€ κ°™λ‹€.

p∣(n2+1)p∣(n+i)(nβˆ’i)

Supp. p is irreducible in Z[i], (κ·€λ₯˜λ²•)

then p would have to divide n+i or nβˆ’i.

If p divides n+i, then n+i=p(a+bi) for some a,b∈Z.

ν—ˆμˆ˜λΆ€μ˜ κ³„μˆ˜λ§Œ λΉ„κ΅ν•˜λ©΄, 1=pbλΌλŠ” 식을 μ–»λŠ”λ° 이것은 λΆˆκ°€λŠ₯ν•˜λ‹€!

λ§ˆμ°¬κ°€μ§€λ‘œ nβˆ’i에 λŒ€ν•΄μ„œλ„ λΆˆκ°€λŠ₯ν•˜λ‹€λŠ” κ²°κ³Όλ₯Ό μ–»λŠ”λ‹€.

λ”°λΌμ„œ μ²˜μŒμ— κ°€μ •ν•œ β€œp is irreducible”은 거짓이닀!


pκ°€ Z[i]μ—μ„œ irreducible이 μ•„λ‹ˆλ―€λ‘œ, p=(a+bi)(c+di)κ°€ λœλ‹€. ((a+bi), (c+di) λͺ¨λ‘ unit이 μ•„λ‹˜!)

μ—¬κΈ°μ„œ norm을 μ·¨ν•˜λ©΄, p2=(a2+b2)(c2+d2) where neither a2+b2=1 nor c2+d2=1.

μ΄λ•Œ, a2+b2κ°€ (a+bi)(aβˆ’bi)둜 factorization λ˜λ―€λ‘œ, p=a2+b2κ°€ λ˜μ–΄ μ„±λ¦½ν•œλ‹€. β—Ό
(p=(a+bi)(c+di)=a2+b2κ°€ λ˜λ―€λ‘œ μ•žμ˜ 쑰건을 λͺ¨λ‘ λ§Œμ‘±ν•œλ‹€!)