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Mod p3 analogues of theorems of Gauss andJacobi on binomial coefficients
John B. Cosgrave1, Karl Dilcher2
1Dublin, Ireland2Dalhousie University, Halifax, Canada
The Fields Institute, September 22, 2009
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 2: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/2.jpg)
We begin with a table:
p( p−1
2p−1
4
)(mod p) a b
5 2 2 1 213 20 7 3 217 70 2 1 429 3432 10 5 237 48620 2 1 641 184756 10 5 453 10400600 39 7 261 10 5 673 67 3 889 10 5 897 18 9 4
p ≡ 1 (mod 4), p = a2 + b2.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 3: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/3.jpg)
Reformulating the table:
p( p−1
2p−1
4
)(mod p) | · · · | < p
2 a b
5 2 2 2 1 213 20 7 −6 3 217 70 2 2 1 429 3432 10 10 5 237 48620 2 2 1 641 184756 10 10 5 453 10400600 39 −14 7 261 10 10 5 673 67 −6 3 889 10 10 5 897 18 18 9 4
p ≡ 1 (mod 4), p = a2 + b2.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 4: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/4.jpg)
1. Introduction
The table is an illustration of the following celebrated result:
Theorem 1 (Gauss, 1828)
Let p ≡ 1 (mod 4) be a prime and write
p = a2 + b2, a ≡ 1 (mod 4).
Then (p−12
p−14
)≡ 2a (mod p).
Several different proofs are known, some using “Jacobsthalsums".
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 5: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/5.jpg)
1. Introduction
The table is an illustration of the following celebrated result:
Theorem 1 (Gauss, 1828)
Let p ≡ 1 (mod 4) be a prime and write
p = a2 + b2, a ≡ 1 (mod 4).
Then (p−12
p−14
)≡ 2a (mod p).
Several different proofs are known, some using “Jacobsthalsums".
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 6: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/6.jpg)
To extend this to a congruence mod p2, we need the conceptof a Fermat quotient : For m ∈ Z, m ≥ 2, and p - m, define
qp(m) :=mp−1 − 1
p.
Beukers (1984) conjectured, and Chowla, Dwork & Evans(1986) proved:
Theorem 2 (Chowla, Dwork, Evans)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a
)(1 + 1
2pqp(2))
(mod p2).
Application: Search for Wilson primes, (p − 1)! ≡ −1 (mod p2).Can this be extended further?
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 7: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/7.jpg)
To extend this to a congruence mod p2, we need the conceptof a Fermat quotient : For m ∈ Z, m ≥ 2, and p - m, define
qp(m) :=mp−1 − 1
p.
Beukers (1984) conjectured, and Chowla, Dwork & Evans(1986) proved:
Theorem 2 (Chowla, Dwork, Evans)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a
)(1 + 1
2pqp(2))
(mod p2).
Application: Search for Wilson primes, (p − 1)! ≡ −1 (mod p2).Can this be extended further?
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 8: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/8.jpg)
To extend this to a congruence mod p2, we need the conceptof a Fermat quotient : For m ∈ Z, m ≥ 2, and p - m, define
qp(m) :=mp−1 − 1
p.
Beukers (1984) conjectured, and Chowla, Dwork & Evans(1986) proved:
Theorem 2 (Chowla, Dwork, Evans)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a
)(1 + 1
2pqp(2))
(mod p2).
Application: Search for Wilson primes, (p − 1)! ≡ −1 (mod p2).
Can this be extended further?
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 9: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/9.jpg)
To extend this to a congruence mod p2, we need the conceptof a Fermat quotient : For m ∈ Z, m ≥ 2, and p - m, define
qp(m) :=mp−1 − 1
p.
Beukers (1984) conjectured, and Chowla, Dwork & Evans(1986) proved:
Theorem 2 (Chowla, Dwork, Evans)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a
)(1 + 1
2pqp(2))
(mod p2).
Application: Search for Wilson primes, (p − 1)! ≡ −1 (mod p2).Can this be extended further?
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 10: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/10.jpg)
2. Interlude: Gauss Factorials
Recall Wilson’s Theorem: p is a prime if and only if
(p − 1)! ≡ −1 (mod p).
Define the Gauss factorial
Nn! =∏
1≤j≤Ngcd(j,n)=1
j .
Theorem 3 (Gauss)For any integer n ≥ 2,
(n − 1)n! ≡
{−1 (mod n) for n = 2,4,pα, or 2pα,
1 (mod n) otherwise,
where p is an odd prime and α is a positive integer.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 11: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/11.jpg)
2. Interlude: Gauss Factorials
Recall Wilson’s Theorem: p is a prime if and only if
(p − 1)! ≡ −1 (mod p).
Define the Gauss factorial
Nn! =∏
1≤j≤Ngcd(j,n)=1
j .
Theorem 3 (Gauss)For any integer n ≥ 2,
(n − 1)n! ≡
{−1 (mod n) for n = 2,4,pα, or 2pα,
1 (mod n) otherwise,
where p is an odd prime and α is a positive integer.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 12: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/12.jpg)
2. Interlude: Gauss Factorials
Recall Wilson’s Theorem: p is a prime if and only if
(p − 1)! ≡ −1 (mod p).
Define the Gauss factorial
Nn! =∏
1≤j≤Ngcd(j,n)=1
j .
Theorem 3 (Gauss)For any integer n ≥ 2,
(n − 1)n! ≡
{−1 (mod n) for n = 2,4,pα, or 2pα,
1 (mod n) otherwise,
where p is an odd prime and α is a positive integer.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 13: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/13.jpg)
Recall Gauss’ Theorem:(p−1
2
)!((
p−14
)!)2 ≡ 2a (mod p).
Can we have something like this for p2 in place of p, usingGauss factorials?
Idea: Use the mod p2 extension by Chowla et al.
Main technical device: We can show that
(p2 − 1
2
)p! ≡ (p − 1)!
p−12
(p − 1
2
)!
1 +p − 1
2p
p−12∑
j=1
1j
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 14: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/14.jpg)
Recall Gauss’ Theorem:(p−1
2
)!((
p−14
)!)2 ≡ 2a (mod p).
Can we have something like this for p2 in place of p, usingGauss factorials?
Idea: Use the mod p2 extension by Chowla et al.
Main technical device: We can show that
(p2 − 1
2
)p! ≡ (p − 1)!
p−12
(p − 1
2
)!
1 +p − 1
2p
p−12∑
j=1
1j
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 15: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/15.jpg)
Recall Gauss’ Theorem:(p−1
2
)!((
p−14
)!)2 ≡ 2a (mod p).
Can we have something like this for p2 in place of p, usingGauss factorials?
Idea: Use the mod p2 extension by Chowla et al.
Main technical device: We can show that
(p2 − 1
2
)p! ≡ (p − 1)!
p−12
(p − 1
2
)!
1 +p − 1
2p
p−12∑
j=1
1j
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 16: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/16.jpg)
Recall Gauss’ Theorem:(p−1
2
)!((
p−14
)!)2 ≡ 2a (mod p).
Can we have something like this for p2 in place of p, usingGauss factorials?
Idea: Use the mod p2 extension by Chowla et al.
Main technical device: We can show that
(p2 − 1
2
)p! ≡ (p − 1)!
p−12
(p − 1
2
)!
1 +p − 1
2p
p−12∑
j=1
1j
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 17: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/17.jpg)
We can derive a similar congruence for(p2 − 1
4
)p! (mod p2).
Also used is the congruence
p−12∑
j=1
1j≡ −2 qp(2) (mod p),
and other similar congruences due to Emma Lehmer (1938)and others before her.
Altogether we have, after simplifying,(p2−1
2
)p!((
p2−14
)p!
)2 ≡(p−1
2p−1
4
)1
1 + 12pqp(2)
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 18: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/18.jpg)
We can derive a similar congruence for(p2 − 1
4
)p! (mod p2).
Also used is the congruence
p−12∑
j=1
1j≡ −2 qp(2) (mod p),
and other similar congruences due to Emma Lehmer (1938)and others before her.
Altogether we have, after simplifying,(p2−1
2
)p!((
p2−14
)p!
)2 ≡(p−1
2p−1
4
)1
1 + 12pqp(2)
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 19: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/19.jpg)
We can derive a similar congruence for(p2 − 1
4
)p! (mod p2).
Also used is the congruence
p−12∑
j=1
1j≡ −2 qp(2) (mod p),
and other similar congruences due to Emma Lehmer (1938)and others before her.
Altogether we have, after simplifying,(p2−1
2
)p!((
p2−14
)p!
)2 ≡(p−1
2p−1
4
)1
1 + 12pqp(2)
(mod p2).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 20: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/20.jpg)
Combining this with the theorem of Chowla, Dwork & Evans:
Theorem 4Let p and a be as before. Then(
p2−12
)p!((
p2−14
)p!
)2 ≡ 2a − p2a (mod p2).
While it would be quite hopeless to conjecture an extension ofthe theorem of Chowla et al., this is easily possible for thetheorem above.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 21: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/21.jpg)
Combining this with the theorem of Chowla, Dwork & Evans:
Theorem 4Let p and a be as before. Then(
p2−12
)p!((
p2−14
)p!
)2 ≡ 2a − p2a (mod p2).
While it would be quite hopeless to conjecture an extension ofthe theorem of Chowla et al., this is easily possible for thetheorem above.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 22: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/22.jpg)
3. Extensions modulo p3
By numerical experimentation we first conjectured
Theorem 5Let p and a be as before. Then(
p3−12
)p!((
p3−14
)p!
)2 ≡ 2a − p2a
− p2
8a3 (mod p3).
(Proof later).
Using more complicated congruences than the ones leading toTheorem 4 (but the same ideas), and going backwards, weobtain
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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3. Extensions modulo p3
By numerical experimentation we first conjectured
Theorem 5Let p and a be as before. Then(
p3−12
)p!((
p3−14
)p!
)2 ≡ 2a − p2a
− p2
8a3 (mod p3).
(Proof later).
Using more complicated congruences than the ones leading toTheorem 4 (but the same ideas), and going backwards, weobtain
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 24: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/24.jpg)
Theorem 6 (Main result)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a− p2
8a3
)×
(1 + 1
2pqp(2) + 18p2
(2Ep−3 − qp(2)2
))(mod p3).
Here Ep−3 is the Euler number defined by
2et + e−t =
∞∑n=0
En
n!tn (|t | < π).
How can we prove Theorem 5?By further experimentation we first conjectured, and thenproved the following generalization.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 25: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/25.jpg)
Theorem 6 (Main result)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a− p2
8a3
)×
(1 + 1
2pqp(2) + 18p2
(2Ep−3 − qp(2)2
))(mod p3).
Here Ep−3 is the Euler number defined by
2et + e−t =
∞∑n=0
En
n!tn (|t | < π).
How can we prove Theorem 5?By further experimentation we first conjectured, and thenproved the following generalization.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 26: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/26.jpg)
Theorem 6 (Main result)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a− p2
8a3
)×
(1 + 1
2pqp(2) + 18p2
(2Ep−3 − qp(2)2
))(mod p3).
Here Ep−3 is the Euler number defined by
2et + e−t =
∞∑n=0
En
n!tn (|t | < π).
How can we prove Theorem 5?
By further experimentation we first conjectured, and thenproved the following generalization.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 27: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/27.jpg)
Theorem 6 (Main result)Let p and a be as before. Then(p−1
2p−1
4
)≡
(2a − p
2a− p2
8a3
)×
(1 + 1
2pqp(2) + 18p2
(2Ep−3 − qp(2)2
))(mod p3).
Here Ep−3 is the Euler number defined by
2et + e−t =
∞∑n=0
En
n!tn (|t | < π).
How can we prove Theorem 5?By further experimentation we first conjectured, and thenproved the following generalization.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 28: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/28.jpg)
Theorem 7Let p and a be as before and let α ≥ 2 be an integer. Then(
pα−12
)p!((
pα−14
)p!
)2 ≡ 2a − 1 · p2a
− 1 · p2
8a3 − 2 · p3
(2a)5 − 5 · p4
(2a)7
− 14 · p5
(2a)9 − . . .− Cα−2pα−1
(2a)2α−1 (mod pα).
Here Cn := 1n+1
(2nn
)is the nth Catalan number which is always
an integer.
Theorem 5 is obviously a special case of Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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Theorem 7Let p and a be as before and let α ≥ 2 be an integer. Then(
pα−12
)p!((
pα−14
)p!
)2 ≡ 2a − 1 · p2a
− 1 · p2
8a3 − 2 · p3
(2a)5 − 5 · p4
(2a)7
− 14 · p5
(2a)9 − . . .− Cα−2pα−1
(2a)2α−1 (mod pα).
Here Cn := 1n+1
(2nn
)is the nth Catalan number which is always
an integer.
Theorem 5 is obviously a special case of Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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Theorem 7Let p and a be as before and let α ≥ 2 be an integer. Then(
pα−12
)p!((
pα−14
)p!
)2 ≡ 2a − 1 · p2a
− 1 · p2
8a3 − 2 · p3
(2a)5 − 5 · p4
(2a)7
− 14 · p5
(2a)9 − . . .− Cα−2pα−1
(2a)2α−1 (mod pα).
Here Cn := 1n+1
(2nn
)is the nth Catalan number which is always
an integer.
Theorem 5 is obviously a special case of Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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4. Main Ingredients in the Proof
• The Jacobi sum
J(χ, ψ) =∑
j mod p
χ(j)ψ(1 − j),
where χ and ψ are characters modulo p.
• Fix a primitive root g mod p;let χ be a character of order 4 such that χ(g) = i .Define integers a′, b′ by
p = a′2 +b′2, a′ ≡(
2p
)(mod 4), b′ ≡ a′g(p−1)/4 (mod p).
These are uniquely defined, differ from a and b of Gauss’theorem only (possibly) in sign.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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4. Main Ingredients in the Proof
• The Jacobi sum
J(χ, ψ) =∑
j mod p
χ(j)ψ(1 − j),
where χ and ψ are characters modulo p.
• Fix a primitive root g mod p;let χ be a character of order 4 such that χ(g) = i .
Define integers a′, b′ by
p = a′2 +b′2, a′ ≡(
2p
)(mod 4), b′ ≡ a′g(p−1)/4 (mod p).
These are uniquely defined, differ from a and b of Gauss’theorem only (possibly) in sign.
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4. Main Ingredients in the Proof
• The Jacobi sum
J(χ, ψ) =∑
j mod p
χ(j)ψ(1 − j),
where χ and ψ are characters modulo p.
• Fix a primitive root g mod p;let χ be a character of order 4 such that χ(g) = i .Define integers a′, b′ by
p = a′2 +b′2, a′ ≡(
2p
)(mod 4), b′ ≡ a′g(p−1)/4 (mod p).
These are uniquely defined, differ from a and b of Gauss’theorem only (possibly) in sign.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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4. Main Ingredients in the Proof
• The Jacobi sum
J(χ, ψ) =∑
j mod p
χ(j)ψ(1 − j),
where χ and ψ are characters modulo p.
• Fix a primitive root g mod p;let χ be a character of order 4 such that χ(g) = i .Define integers a′, b′ by
p = a′2 +b′2, a′ ≡(
2p
)(mod 4), b′ ≡ a′g(p−1)/4 (mod p).
These are uniquely defined, differ from a and b of Gauss’theorem only (possibly) in sign.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• Then
J(χ, χ) = (−1)p−1
4 (a′ + ib′),
J(χ3, χ3) = (−1)p−1
4 (a′ − ib′),
• On the other hand,
J(χ, χ) ≡ 0 (mod p),
J(χ3, χ3) =Γp(1 − 1
2)
Γp(1 − 14)2
.
These are deep results, related to the “Gross-Koblitz formula"(see, e.g., Gauss and Jacobi Sums by B. Berndt, R. Evans andK. Williams).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• Then
J(χ, χ) = (−1)p−1
4 (a′ + ib′),
J(χ3, χ3) = (−1)p−1
4 (a′ − ib′),
• On the other hand,
J(χ, χ) ≡ 0 (mod p),
J(χ3, χ3) =Γp(1 − 1
2)
Γp(1 − 14)2
.
These are deep results, related to the “Gross-Koblitz formula"(see, e.g., Gauss and Jacobi Sums by B. Berndt, R. Evans andK. Williams).
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• Γp(z) is the p-adic gamma function defined by
F (n) := (−1)n∏
0<j<np-j
j ,
Γp(z) = limn→z
F (n) (z ∈ Zp),
where n runs through any sequence of positive integersp-adically approaching z.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• In particular,
(−1)p−1
4 (a′ − ib′) = J(χ3, χ3) =Γp(1 − 1
2)
Γp(1 − 14)2
≡Γp(1 + pα−1
2 )
Γp(1 + pα−14 )2
(mod pα)
=F (1 + pα−1
2 )
F (1 + pα−14 )2
= −
(pα−1
2
)p!((
pα−14
)p!
)2 .
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• Raise
(−1)p−1
4 (a′ + ib′) = J(χ, χ) ≡ 0 (mod p)
to the power α:
(a′ + ib′)α ≡ 0 (mod pα).
• Expand the left-hand side; get binomial coefficients;
• separate real and imaginary parts;
• use the combinatorial identity (k = 0,1, . . . ,n − 1)
k∑j=0
(−1)j
j + 1
(2jj
)(n + j − k
k − j
)=
(n − 1 − k
k
);
• putting everything together, we obtain Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• Raise
(−1)p−1
4 (a′ + ib′) = J(χ, χ) ≡ 0 (mod p)
to the power α:
(a′ + ib′)α ≡ 0 (mod pα).
• Expand the left-hand side; get binomial coefficients;
• separate real and imaginary parts;
• use the combinatorial identity (k = 0,1, . . . ,n − 1)
k∑j=0
(−1)j
j + 1
(2jj
)(n + j − k
k − j
)=
(n − 1 − k
k
);
• putting everything together, we obtain Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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• Raise
(−1)p−1
4 (a′ + ib′) = J(χ, χ) ≡ 0 (mod p)
to the power α:
(a′ + ib′)α ≡ 0 (mod pα).
• Expand the left-hand side; get binomial coefficients;
• separate real and imaginary parts;
• use the combinatorial identity (k = 0,1, . . . ,n − 1)
k∑j=0
(−1)j
j + 1
(2jj
)(n + j − k
k − j
)=
(n − 1 − k
k
);
• putting everything together, we obtain Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 42: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/42.jpg)
• Raise
(−1)p−1
4 (a′ + ib′) = J(χ, χ) ≡ 0 (mod p)
to the power α:
(a′ + ib′)α ≡ 0 (mod pα).
• Expand the left-hand side; get binomial coefficients;
• separate real and imaginary parts;
• use the combinatorial identity (k = 0,1, . . . ,n − 1)
k∑j=0
(−1)j
j + 1
(2jj
)(n + j − k
k − j
)=
(n − 1 − k
k
);
• putting everything together, we obtain Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 43: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/43.jpg)
• Raise
(−1)p−1
4 (a′ + ib′) = J(χ, χ) ≡ 0 (mod p)
to the power α:
(a′ + ib′)α ≡ 0 (mod pα).
• Expand the left-hand side; get binomial coefficients;
• separate real and imaginary parts;
• use the combinatorial identity (k = 0,1, . . . ,n − 1)
k∑j=0
(−1)j
j + 1
(2jj
)(n + j − k
k − j
)=
(n − 1 − k
k
);
• putting everything together, we obtain Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 44: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/44.jpg)
• Raise
(−1)p−1
4 (a′ + ib′) = J(χ, χ) ≡ 0 (mod p)
to the power α:
(a′ + ib′)α ≡ 0 (mod pα).
• Expand the left-hand side; get binomial coefficients;
• separate real and imaginary parts;
• use the combinatorial identity (k = 0,1, . . . ,n − 1)
k∑j=0
(−1)j
j + 1
(2jj
)(n + j − k
k − j
)=
(n − 1 − k
k
);
• putting everything together, we obtain Theorem 7.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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5. A Jacobi Analogue
Let p ≡ 1 (mod 6). Then we can write
4p = r2 + 3s2, r ≡ 1 (mod 3), 3 | s,
which determines r uniquely.
In analogy to Gauss’ Theorem 1 we have
Theorem 8 (Jacobi, 1837)Let p and r be as above. Then(2(p−1)
3p−1
3
)≡ −r (mod p).
This was generalized to mod p2 independently by Evans(unpublished, 1985) and Yeung (1989):
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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5. A Jacobi Analogue
Let p ≡ 1 (mod 6). Then we can write
4p = r2 + 3s2, r ≡ 1 (mod 3), 3 | s,
which determines r uniquely.
In analogy to Gauss’ Theorem 1 we have
Theorem 8 (Jacobi, 1837)Let p and r be as above. Then(2(p−1)
3p−1
3
)≡ −r (mod p).
This was generalized to mod p2 independently by Evans(unpublished, 1985) and Yeung (1989):
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 47: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/47.jpg)
5. A Jacobi Analogue
Let p ≡ 1 (mod 6). Then we can write
4p = r2 + 3s2, r ≡ 1 (mod 3), 3 | s,
which determines r uniquely.
In analogy to Gauss’ Theorem 1 we have
Theorem 8 (Jacobi, 1837)Let p and r be as above. Then(2(p−1)
3p−1
3
)≡ −r (mod p).
This was generalized to mod p2 independently by Evans(unpublished, 1985) and Yeung (1989):
John B. Cosgrave, Karl Dilcher Mod p3 analogues
![Page 48: Mod p3 analogues of theorems of Gauss and Jacobi on ... · PDF fileMod p3 analogues of theorems of Gauss and Jacobi on binomial ... Search for Wilson ... To extend this to a congruence](https://reader033.vdocuments.us/reader033/viewer/2022051523/5a7885397f8b9ae91b8b7e94/html5/thumbnails/48.jpg)
Theorem 9 (Evans; Yeung)Let p and r be as above. Then(2(p−1)
3p−1
3
)≡ −r +
pr
(mod p2).
With methods similar to those in the first part of this talk, weproved
Theorem 10Let p and r be as above. Then(2(p−1)
3p−1
3
)≡
(−r +
pr
+p2
r3
) (1 +
16
p2Bp−2(13)
)(mod p3).
Here Bn(x) is the nth Bernoulli polynomial.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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Theorem 9 (Evans; Yeung)Let p and r be as above. Then(2(p−1)
3p−1
3
)≡ −r +
pr
(mod p2).
With methods similar to those in the first part of this talk, weproved
Theorem 10Let p and r be as above. Then(2(p−1)
3p−1
3
)≡
(−r +
pr
+p2
r3
) (1 +
16
p2Bp−2(13)
)(mod p3).
Here Bn(x) is the nth Bernoulli polynomial.
John B. Cosgrave, Karl Dilcher Mod p3 analogues
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Thank you
John B. Cosgrave, Karl Dilcher Mod p3 analogues