Input really must be positive definite, not just definite with any sign.
Function: qflllgram
Section: linear_algebra
C-Name: qflllgram0
Prototype: GD0,L,
Help: qflllgram(G,{flag=0}): LLL reduction of the lattice whose gram matrix
is G (gives the unimodular transformation matrix). flag is optional and can
be 0: default,1: assumes x is integral, 4: assumes x is integral,
returns [K,T], where K is the integer kernel of x
and T the LLL reduced image, 5: same as 4 but x may have polynomial
coefficients, 8: same as 0 but x may have polynomial coefficients.
Doc: same as \kbd{qflll}, except that the
matrix $G = \kbd{x\til * x}$ is the Gram matrix of some lattice vectors
$x$,
and not the coordinates of the vectors themselves. In particular, $G$ must
now be a square symmetric real matrix, corresponding to a positive
quadratic form (not necessarily definite: $x$ needs not have maximal rank).
The result is a unimodular
transformation matrix $T$ such that $x \cdot T$ is an LLL-reduced basis of
the lattice generated by the column vectors of $x$. See \tet{qflll} for
further details about the LLL implementation.
If $\fl=0$ (default), assume that $G$ has either exact (integral or
rational) or real floating point entries. The matrix is rescaled, converted
to integers and the behavior is then as in $\fl = 1$.
If $\fl=1$, assume that $G$ is integral. Computations involving
Gram-Schmidt
vectors are approximate, with precision varying as needed (Lehmer's trick,
as generalized by Schnorr). Adapted from Nguyen and Stehl\'e's algorithm
and Stehl\'e's code (\kbd{fplll-1.3}).
$\fl=4$: $G$ has integer entries, gives the kernel and reduced image of
$x$.
$\fl=5$: same as $4$, but $G$ may have polynomial coefficients.
Variant: Also available are \fun{GEN}{lllgram}{GEN G} ($\fl=0$),
\fun{GEN}{lllgramint}{GEN G} ($\fl=1$), and \fun{GEN}{lllgramkerim}{GEN G}
($\fl=4$).
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