Dual groups of Finite Multiplicative Abelian Groups¶
The basic idea is very simple. Let G be an abelian group and \(G^*\) its dual (i.e., the group of homomorphisms from G to \(\CC^\times\)). Let \(g_j\), \(j=1,..,n\), denote generators of \(G\) - say \(g_j\) is of order \(m_j>1\). There are generators \(X_j\), \(j=1,..,n\), of \(G^*\) for which \(X_j(g_j)=\exp(2\pi i/m_j)\) and \(X_i(g_j)=1\) if \(i\not= j\). These are used to construct \(G^*\).
Sage supports multiplicative abelian groups on any prescribed finite
number \(n > 0\) of generators. Use
AbelianGroup()
function
to create an abelian group, the
dual_group()
method to create its dual, and then the gen()
and gens()
methods to obtain the corresponding generators. You can print the
generators as arbitrary strings using the optional names
argument
to the
dual_group()
method.
EXAMPLES:
sage: F = AbelianGroup(5, [2,5,7,8,9], names='abcde')
sage: (a, b, c, d, e) = F.gens()
sage: Fd = F.dual_group(names='ABCDE')
sage: Fd.base_ring()
Cyclotomic Field of order 2520 and degree 576
sage: A,B,C,D,E = Fd.gens()
sage: A(a)
-1
sage: A(b), A(c), A(d), A(e)
(1, 1, 1, 1)
sage: Fd = F.dual_group(names='ABCDE', base_ring=CC)
sage: A,B,C,D,E = Fd.gens()
sage: A(a) # abs tol 1e-8
-1.00000000000000 + 0.00000000000000*I
sage: A(b); A(c); A(d); A(e)
1.00000000000000
1.00000000000000
1.00000000000000
1.00000000000000
AUTHORS:
- David Joyner (2006-08) (based on abelian_groups)
- David Joyner (2006-10) modifications suggested by William Stein
- Volker Braun (2012-11) port to new Parent base. Use tuples for immutables. Default to cyclotomic base ring.
-
sage.groups.abelian_gps.dual_abelian_group.
DualAbelianGroup_class
¶ Dual of abelian group.
EXAMPLES:
sage: F = AbelianGroup(5,[3,5,7,8,9], names="abcde") sage: F.dual_group() Dual of Abelian Group isomorphic to Z/3Z x Z/5Z x Z/7Z x Z/8Z x Z/9Z over Cyclotomic Field of order 2520 and degree 576 sage: F = AbelianGroup(4,[15,7,8,9], names="abcd") sage: F.dual_group(base_ring=CC) Dual of Abelian Group isomorphic to Z/15Z x Z/7Z x Z/8Z x Z/9Z over Complex Field with 53 bits of precision
-
sage.groups.abelian_gps.dual_abelian_group.
is_DualAbelianGroup
(x)¶ Return True if \(x\) is the dual group of an abelian group.
EXAMPLES:
sage: from sage.groups.abelian_gps.dual_abelian_group import is_DualAbelianGroup sage: F = AbelianGroup(5,[3,5,7,8,9], names=list("abcde")) sage: Fd = F.dual_group() sage: is_DualAbelianGroup(Fd) True sage: F = AbelianGroup(3,[1,2,3], names='a') sage: Fd = F.dual_group() sage: Fd.gens() (1, X1, X2) sage: F.gens() (1, a1, a2)