Teoría de Números

Sage tiene bastante funcionalidad para teoría de números. Por ejemplo, podemos hacer aritmética en \(\ZZ/N\ZZ\) del modo siguiente:

sage: R = IntegerModRing(97)
sage: a = R(2) / R(3)
sage: a
33
sage: a.rational_reconstruction()
2/3
sage: b = R(47)
sage: b^20052005
50
sage: b.modulus()
97
sage: b.is_square()
True

>>> from sage.all import *
>>> R = IntegerModRing(Integer(97))
>>> a = R(Integer(2)) / R(Integer(3))
>>> a
33
>>> a.rational_reconstruction()
2/3
>>> b = R(Integer(47))
>>> b**Integer(20052005)
50
>>> b.modulus()
97
>>> b.is_square()
True

Sage contiene las funciones estándar de teoría de números. Por ejemplo:

sage: gcd(515,2005)
5
sage: factor(2005)
5 * 401
sage: c = factorial(25); c
15511210043330985984000000
sage: [valuation(c,p) for p in prime_range(2,23)]
[22, 10, 6, 3, 2, 1, 1, 1]
sage: next_prime(2005)
2011
sage: previous_prime(2005)
2003
sage: divisors(28); sum(divisors(28)); 2*28
[1, 2, 4, 7, 14, 28]
56
56

>>> from sage.all import *
>>> gcd(Integer(515),Integer(2005))
5
>>> factor(Integer(2005))
5 * 401
>>> c = factorial(Integer(25)); c
15511210043330985984000000
>>> [valuation(c,p) for p in prime_range(Integer(2),Integer(23))]
[22, 10, 6, 3, 2, 1, 1, 1]
>>> next_prime(Integer(2005))
2011
>>> previous_prime(Integer(2005))
2003
>>> divisors(Integer(28)); sum(divisors(Integer(28))); Integer(2)*Integer(28)
[1, 2, 4, 7, 14, 28]
56
56

¡Un número perfecto!

La función sigma(n,k) suma las potencias \(k\)-ésimas de los divisores de n:

sage: sigma(28,0); sigma(28,1); sigma(28,2)
6
56
1050

>>> from sage.all import *
>>> sigma(Integer(28),Integer(0)); sigma(Integer(28),Integer(1)); sigma(Integer(28),Integer(2))
6
56
1050

A continuación ilustramos el algoritmo de Euclides extendido, la función \(\phi\) de Euler, y la función prime_to_m_part(n, m), que devuelve el mayor divisor de n que es primo relativo a m:

sage: d,u,v = xgcd(12,15)
sage: d == u*12 + v*15
True
sage: n = 2005
sage: inverse_mod(3,n)
1337
sage: 3 * 1337
4011
sage: prime_divisors(n)
[5, 401]
sage: phi = n*prod([1 - 1/p for p in prime_divisors(n)]); phi
1600
sage: euler_phi(n)
1600
sage: prime_to_m_part(n, 5)
401

>>> from sage.all import *
>>> d,u,v = xgcd(Integer(12),Integer(15))
>>> d == u*Integer(12) + v*Integer(15)
True
>>> n = Integer(2005)
>>> inverse_mod(Integer(3),n)
1337
>>> Integer(3) * Integer(1337)
4011
>>> prime_divisors(n)
[5, 401]
>>> phi = n*prod([Integer(1) - Integer(1)/p for p in prime_divisors(n)]); phi
1600
>>> euler_phi(n)
1600
>>> prime_to_m_part(n, Integer(5))
401

Seguimos con un ejemplo sobre el problema \(3n+1\):

sage: n = 2005
sage: for i in range(1000):
....:     n = 3*odd_part(n) + 1
....:     if odd_part(n)==1:
....:         print(i)
....:         break
38

>>> from sage.all import *
>>> n = Integer(2005)
>>> for i in range(Integer(1000)):
...     n = Integer(3)*odd_part(n) + Integer(1)
...     if odd_part(n)==Integer(1):
...         print(i)
...         break
38

Finalmente ilustramos el teorema chino del resto:

sage: x = crt(2, 1, 3, 5); x
11
sage: x % 3  # x mod 3 = 2
2
sage: x % 5  # x mod 5 = 1
1
sage: [binomial(13,m) for m in range(14)]
[1, 13, 78, 286, 715, 1287, 1716, 1716, 1287, 715, 286, 78, 13, 1]
sage: [binomial(13,m)%2 for m in range(14)]
[1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1]
sage: [kronecker(m,13) for m in range(1,13)]
[1, -1, 1, 1, -1, -1, -1, -1, 1, 1, -1, 1]
sage: n = 10000; sum([moebius(m) for m in range(1,n)])
-23
sage: Partitions(4).list()
[[4], [3, 1], [2, 2], [2, 1, 1], [1, 1, 1, 1]]

>>> from sage.all import *
>>> x = crt(Integer(2), Integer(1), Integer(3), Integer(5)); x
11
>>> x % Integer(3)  # x mod 3 = 2
2
>>> x % Integer(5)  # x mod 5 = 1
1
>>> [binomial(Integer(13),m) for m in range(Integer(14))]
[1, 13, 78, 286, 715, 1287, 1716, 1716, 1287, 715, 286, 78, 13, 1]
>>> [binomial(Integer(13),m)%Integer(2) for m in range(Integer(14))]
[1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1]
>>> [kronecker(m,Integer(13)) for m in range(Integer(1),Integer(13))]
[1, -1, 1, 1, -1, -1, -1, -1, 1, 1, -1, 1]
>>> n = Integer(10000); sum([moebius(m) for m in range(Integer(1),n)])
-23
>>> Partitions(Integer(4)).list()
[[4], [3, 1], [2, 2], [2, 1, 1], [1, 1, 1, 1]]

Números p-ádicos

El cuerpo de números p-ádicos está implementado en Sage. Observa que una vez creamos un cuerpo \(p\)-ádico, no podemos cambiar su precisión:

sage: K = Qp(11); K
11-adic Field with capped relative precision 20
sage: a = K(211/17); a
4 + 4*11 + 11^2 + 7*11^3 + 9*11^5 + 5*11^6 + 4*11^7 + 8*11^8 + 7*11^9
+ 9*11^10 + 3*11^11 + 10*11^12 + 11^13 + 5*11^14 + 6*11^15 + 2*11^16
+ 3*11^17 + 11^18 + 7*11^19 + O(11^20)
sage: b = K(3211/11^2); b
10*11^-2 + 5*11^-1 + 4 + 2*11 + O(11^18)

>>> from sage.all import *
>>> K = Qp(Integer(11)); K
11-adic Field with capped relative precision 20
>>> a = K(Integer(211)/Integer(17)); a
4 + 4*11 + 11^2 + 7*11^3 + 9*11^5 + 5*11^6 + 4*11^7 + 8*11^8 + 7*11^9
+ 9*11^10 + 3*11^11 + 10*11^12 + 11^13 + 5*11^14 + 6*11^15 + 2*11^16
+ 3*11^17 + 11^18 + 7*11^19 + O(11^20)
>>> b = K(Integer(3211)/Integer(11)**Integer(2)); b
10*11^-2 + 5*11^-1 + 4 + 2*11 + O(11^18)

Se ha hecho mucho trabajo para implementar otros anillos de enteros sobre cuerpos p-ádicos. El lector interesado está invitado a pedir más detalles a los expertos en el grupo de Google sage-support.

Varios métodos relacionados están implementados en la clase NumberField:

sage: R.<x> = PolynomialRing(QQ)
sage: K = NumberField(x^3 + x^2 - 2*x + 8, 'a')
sage: K.integral_basis()
[1, 1/2*a^2 + 1/2*a, a^2]

>>> from sage.all import *
>>> R = PolynomialRing(QQ, names=('x',)); (x,) = R._first_ngens(1)
>>> K = NumberField(x**Integer(3) + x**Integer(2) - Integer(2)*x + Integer(8), 'a')
>>> K.integral_basis()
[1, 1/2*a^2 + 1/2*a, a^2]

sage: K.galois_group()
Galois group 3T2 (S3) with order 6 of x^3 + x^2 - 2*x + 8

>>> from sage.all import *
>>> K.galois_group()
Galois group 3T2 (S3) with order 6 of x^3 + x^2 - 2*x + 8

sage: K.polynomial_quotient_ring()
Univariate Quotient Polynomial Ring in a over Rational Field with modulus
x^3 + x^2 - 2*x + 8
sage: K.units()
(-3*a^2 - 13*a - 13,)
sage: K.discriminant()
-503
sage: K.class_group()
Class group of order 1 of Number Field in a with defining polynomial x^3 + x^2 - 2*x + 8
sage: K.class_number()
1

>>> from sage.all import *
>>> K.polynomial_quotient_ring()
Univariate Quotient Polynomial Ring in a over Rational Field with modulus
x^3 + x^2 - 2*x + 8
>>> K.units()
(-3*a^2 - 13*a - 13,)
>>> K.discriminant()
-503
>>> K.class_group()
Class group of order 1 of Number Field in a with defining polynomial x^3 + x^2 - 2*x + 8
>>> K.class_number()
1