charorder

Function: charorder
Section: number_theoretical
C-Name: charorder0
Prototype: GG
Help: charorder(cyc,chi): given a finite abelian group (by its elementary
 divisors cyc) and a character chi, return the order of chi.
Doc: let \var{cyc} represent a finite abelian group by its elementary
 divisors, i.e. $(d_j)$ represents $\sum_{j \leq k} \Z/d_j\Z$ with $d_k
 \mid \dots \mid d_1$; any object which has a \kbd{.cyc} method is also
 allowed, e.g.~the output of \kbd{znstar} or \kbd{bnrinit}. A character
 on this group is given by a row vector $\chi = [a_1,\ldots,a_n]$ such that
 $\chi(\prod g_j^{n_j}) = \exp(2\pi i\sum a_j n_j / d_j)$, where $g_j$ denotes
 the generator (of order $d_j$) of the $j$-th cyclic component.

 This function returns the order of the character \kbd{chi}.
 \bprog
 ? cyc = [15,5]; chi = [1,1];
 ? charorder(cyc, chi)
 %2 = 15
 ? bnf = bnfinit(x^2+23);
 ? bnf.cyc
 %4 = [3]
 ? charorder(bnf, [1])
 %5 = 3
 @eprog\noindent For Dirichlet characters (when \kbd{cyc} is
 \kbd{znstar(q, 1)}), characters in Conrey representation are available,
 see \secref{se:dirichletchar} or \kbd{??character}:
 \bprog
 ? G = znstar(100, 1); \\ (Z/100Z)^*
 ? charorder(G, 7)   \\ Conrey label
 %2 = 4
 @eprog

Variant: Also available is
 \fun{GEN}{charorder}{GEN cyc, GEN chi}, when \kbd{cyc} is known to
 be a vector of elementary divisors and \kbd{chi} a compatible character
 (no checks).