bestapprnf

Function: bestapprnf
Section: linear_algebra
C-Name: bestapprnf
Prototype: GGDGp
Help: bestapprnf(V,T,{rootT}): T being an integral polynomial
 and V being a scalar, vector, or matrix, return a reasonable
 approximation of V with polmods modulo T. The rootT argument,
 if present, must be an element of polroots(T), i.e. a root of T fixing a
 complex embedding of Q[x]/(T).
Doc: $T$ being an integral polynomial and $V$ being a scalar, vector, or
 matrix with complex coefficients, return a reasonable approximation of $V$
 with polmods modulo $T$. $T$ can also be any number field structure, in which
 case the minimal polynomial attached to the structure (\kbd{$T$}.pol) is
 used. The \var{rootT} argument, if present, must be an element of
 \kbd{polroots($T$)} (or \kbd{$T$}.pol), i.e.~a complex root of $T$ fixing an embedding of
 $\Q[x]/(T)$ into $\C$.
 \bprog
 ? bestapprnf(sqrt(5), polcyclo(5))
 %1 = Mod(-2*x^3 - 2*x^2 - 1, x^4 + x^3 + x^2 + x + 1)
 ? bestapprnf(sqrt(5), polcyclo(5), exp(4*I*Pi/5))
 %2 = Mod(2*x^3 + 2*x^2 + 1, x^4 + x^3 + x^2 + x + 1)
 @eprog\noindent When the output has huge rational coefficients, try to
 increase the working \kbd{realbitprecision}: if the answer does not
 stabilize, consider that the reconstruction failed.
 Beware that if $T$ is not Galois over $\Q$, some embeddings
 may not allow to reconstruct $V$:
 \bprog
 ? T = x^3-2; vT = polroots(T); z = 3*2^(1/3)+1;
 ? bestapprnf(z, T, vT[1])
 %2 = Mod(3*x + 1, x^3 - 2)
 ? bestapprnf(z, T, vT[2])
 %3 = 4213714286230872/186454048314072  \\ close to 3*2^(1/3) + 1
 @eprog