Primes Divisible by 3 with a Remainder of 1: Necessary and Sufficient Conditions
📂Number TheoryPrimes Divisible by 3 with a Remainder of 1: Necessary and Sufficient Conditions
Theorem
p=3 is a prime number.
p≡1(mod3) ⟺ for some a,b∈Z p=a2−ab+b2
Description
Though p=3 is excluded, in fact since 3=22−2⋅1+12, it wouldn’t matter much if it was included in the theorem.
For example, 13≡1(mod4) is
13=1−4+16=12−1⋅4+42
37≡1(mod4) is
37=9−21+49=32−3⋅7+72
61≡1(mod4) is
61=16−36+81=42−4⋅9+92
These facts are interesting in themselves, but also have deep connections with Eisenstein integers, taking number theory to a higher dimension.
Proof
(⟹)
a2+3b2=(a+b)2−(a+b)(a−b)+(a−b)2
Thus, it suffices to show the existence of a,b∈Z that satisfies p=a2+3b2.
Part 1.
Since p≡1(mod3) is odd, for some k∈N, it can be expressed as p=6k+1.
Part 1-1. Gauss’s Law of Quadratic Reciprocity
Gauss’s Law of Quadratic Reciprocity: For two different odd numbers p,q,
- [2]: (qp)(pq)=(−1)2p−12q−1
By Gauss’s law of quadratic reciprocity,
(3p)(p3)====(−1)2p−123−1(−1)2(6k+1)−1(−1)3k(−1)k
Part 1-2. Euler’s Criterion
Euler’s Criterion: a2p−1≡(pa)(modp)
By Euler’s criterion,
(p−1)≡(−1)2p−1≡(−1)k(modp)
Part 1-3. Multiplicative Property of the Legendre Symbol
Multiplicative Property of the Legendre Symbol: For a prime number p larger than 2, (pab)=(pa)(pb)
By the multiplicative property of the Legendre symbol,
(p3)=(p−1)(p−3)
Since p≡1(mod3), there must exist x=1 satisfying x2≡p(mod3),
(3p)=1
Hence, we obtain the following:
(3p)(p3)=(p−1)(p−3)
Part 1-4.
- Case 1. If k is even
- By Part 1-1, (3p)(p3)=1
- By Part 1-2, (p−1)=1
- By Part 1-3, 1=1⋅(p−3)
- Case 2. If k is odd
- By Part 1-1, since (3p)(p3)=−1, (3p)=−(p3)
- By Part 1-2, since (p−1)=−1, (3p)=(p−1)(p3)=(p−3)
- By Part 1-3, 1=(p−3)
In any case, −3 is a quadratic residue of p, and there exists c∈Z satisfying c2≡−3(modp). Multiplying both sides by B2 and rearranging gives
(cB)2+3B2≡0(modp)
and, if set A:=(cB), for some M∈Z,
A2+3B2=Mp
Also,
M=pA2+3B2≤p(p−1)2−3⋅12=p−p2p−4<p
thus M<p. By continuously reducing M to M=1, for some a,b∈Z, p=a2+3b2 can be said.
Part 2. 1≤r<M
u≡A(modM)v≡B(modM)−2M≤u,v≤2M
Considering u,v∈Z that satisfies (for instance, if M=13 and A≡10(mod13), then u≡−3(mod13), its existence is always assured). Then, since A2+3B2=pM in Part 1,
u2+3v2≡A2+3B2≡0(modM)
and for some r∈Z, u2+3v2=rM.
Part 2-1. r<M
r=Mu2+3v2≤M(M/2)2+3(M/2)2≤M
Therefore, r≤M.
In the case of M=r, the only possibility is when 2M=∣u∣=∣v∣,
u2+3v2=4u2=4v2=M2
- Here, since M=∣2u∣ is even, for some α∈Z, u=Mα+A=2∣u∣α+A so, u≡A(mod2).
- Similarly, M=∣2v∣ and, for some β∈Z, v=Mβ+B=2∣v∣β+B so, v≡B(mod2).
Summarizing, u and A must both be even or odd, and v and B must also be either both even or odd. Let’s check if the following equation can be satisfied.
A2+3B2=Mp=2∣u∣p
- Case 1. Both A and B are even
Since A is even, ∣u∣ is also even and can be divided by 2. Dividing both sides of A2+3B2=2∣u∣p by 4 yields
(2A)2+3(2B)2=(2∣u∣)p
This excessively presupposes A,B,M in Part 1. A new
A′:=(2A)B′:=(2B)M’:=(2∣u∣)
should be set and Part 2 started again. - Case 2. A is even, B is odd
The left side of A2+3B2=2∣u∣p is odd while the right side is even, so equation A2+3B2=Mp cannot hold. - Case 3. A is odd, B is even
The left side of A2+3B2=2∣u∣p is odd while the right side is even, so equation A2+3B2=Mp cannot hold. - Case 4. Both A and B are odd
Let n,m∈Z, A:=2n+1, B:=2m+1,
A2+3B3=4n2+4n+1+3(4m2+4m+1)=4(n2+n+3m2+3m+1)
is a multiple of 4. However, since both ∣u∣ and p are odd, Mp=2∣u∣p cannot be a multiple of 4, and equation A2+3B2=Mp cannot hold.
After examining all possible cases, Cases 2 to 4 show that r<M can be inferred.
Part 2-2. 1≤r
Assuming r=0, since {0=u≡A(modM)0=v≡B(modM), A,B must be a multiple of M, and A2+3B2 must be a multiple of M2. Therefore, M must be a multiple of p, but since Part 1 showed M<p, this is impossible. Thus, r must be larger than 0.
Combining Part 2-1 and Part 2-2, we obtain 1≤r<M.
Part 3. Modified Cauchy-Schwarz Inequality
===(u2+3v2)(A2+3B2)u2A2+3v2A2+3u2B2+9v2B2(u2A2+6uAvB+3v2B2)+(3v2A2−6uAvB+3u2B2)(uA+3vB)2+3(uA−vB)2
Meanwhile,
3(uA−vB)≡3(BA−AB)≡0(modM)
therefore, 3(uA−vB) is a multiple of M. From Part 2,
(uA+3vB)≡AA+3BB≡0(modM)
thus, (uA+3vB) is a multiple of M.
Part 4.
Since A2+3B2=Mp and u2+3v2=Mr,
(u2+3v2)(A2+3B2)=M2rp
Using the modified Cauchy-Schwarz inequality,
(uA+3vB)2+3(uA−vB)2=M2rp
Since (uA+3vB) and 3(uA−vB) from Part 3 are multiples of M, dividing both sides by M2 yields
(MuA+3vB)2+3(MuA−vB)2=rp
With a new
A2:=(MuA+3vB)B2:=(MuA−vB)M2:=r
defined, we regain A22+3B22=M2p. Thus, such Ak,Bk,Mk as defined in Parts 1~3 possesses the same properties for k∈N. Since r<M in Part 2, Mk decreases with each increment of k, stopping exactly at M=1.
(⟸)
Let the quotient of a,b satisfying p=a2−ab+b2 divided by 3 be n,m.
- If N0=3k+0,
N02≡9k2≡0(mod3)
- If N1=3k+1,
N12≡(9k2+6k)+1≡1(mod3)
- If N2=3k+2,
N02≡(9k2+12k+3)+1≡1(mod3)
Therefore, for a natural number N not a multiple of 3 in regard to (mod3), any way N2≡1(mod3) is achieved.
From now, let’s consider all possible combinations of a, b.
- Case 00. a=3n, b=3m
p=9n2−9nm+9m2=3(3n2−3nm+3m2)
The prime number p becomes a multiple of 3, thus this combination is impossible.
- Case 01. a=3n, b=3m+1
p≡9n2−3nb+b2≡1(mod3)
- Case 02. a=3n, b=3m+2
p≡9n2−3nb+b2≡1(mod3)
- Case 11. a=3n+1, b=3m+1
p≡a2−(3n+1)(3m+1)+b2≡1−9nm−3n−3m−1+1≡1(mod3)
- Case 12. a=3n+1, b=3m+2
p===(3n+1)2−(3n+1)(3m+2)+(3m+2)29n2+6n+1−9nm−6n−3m−2+9m2+12m+43(3n2+3m2−3nm+2n+4m+1)
The prime number p becomes a multiple of 3, thus this combination is impossible.
- Case 22. a=3n+2, b=3m+2
p≡≡≡≡a2−(3n+2)(3m+2)+n21−9nm−6n−6m−4+1−21(mod3)
Reviewing all cases, Case 01, Case 02, Case 11, and Case 22 occur when p≡1(mod3), and Case 00, Case 12 do not occur.
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See Also