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Solving systems of polynomial inequalities over real closed fields in subexponential time
N. N. Vorobjov (Jr.),
D. Yu. Grigor'ev
Abstract:
Let
$\mathbb{Q}_m$ denote the field
$\mathbb{Q}(\delta_1,\dots,\delta_m)$, where
$m\geqslant1$,
$\mathbb{Q}_0=\mathbb{Q}$ and an element
$\delta_m$ is infinitesimal relatively to the
elements of the field
$\mathbb{Q}_{m-1}$, i.e.
$0<\delta_m<a$ for any
$0<a\in\mathbb{Q}_{m-1}$. Denote by
$\widetilde{\mathbb{Q}}_m$ the real closure of
$\mathbb{Q}_m$.
A subexponential-time algorithm is described for finding solutions
in
$\widetilde{\mathbb{Q}}_m$ of a system of polynomial inequalities. In the case
$m=0$ (i.e.
$\mathbb{Q}_m=\mathbb{Q}$), exponential-time procedures were designed
for this problem ([4], [16], [20]). Finally, in [3], [21]
the authors proposed subexponential-time algorithm for
$m=0$.
Let
$$
f_1>0,\dots,f_{k_1}>0, f_{k_1+1}\geqslant0,\dots,f_k\geqslant0 \qquad{(1)}
$$
be the input system where polynomials
$f_1,\dots,f_k\in\mathbb{Q}_m[X_1,\dots,X_n]$.
Denote by
$V\subset(\widetilde{\mathbb{Q}}_m)^n$ the set of all solutions of (1). For a rational
function $\alpha=\alpha_1/\alpha_2\in\mathbb{Q}_m(X_1,\dots X_n)$ where polynomials
$\alpha_1,\alpha_2\in\mathbb{Z}[\delta_1,\dots,\delta_m][X_1,\dots,X_n]$ denote by
$l(\alpha)$ the maximum bit lengths
of the integer coefficients of
$\alpha_1$,
$\alpha_2$. Suppose, that the following inequalities are valid:
$$
\mathrm{deg}_{\delta_1,\dots,\delta_m}(f_i)<d_0,\quad \mathrm{deg}_{X_1,\dots,X_n}(f_i)<d,\quad l(f_i)\leqslant M,\quad1\leqslant i\leqslant k. \qquad{(2)}
$$
The notation
$h_1\leqslant\mathcal{P}(h_2,\dots,h_t)$ for functions
$h_1>0,\dots,h_t>0$
means that for suitable natural numbers
$p$,
$q$ an inequality
$h_1\leqslant p(h_2\cdots h_t)^q$ is true.
THEOREM. There is an algorithm which for system (1), satisfying (2),
produces
$a$ finite set
$\mathcal{J}\subset V$ having nonempty intersection
with each component of connectivity of
$V$, with the number
of points in
$\mathcal{J}$ not exceeding
$\mathcal{P}((kd)^{n^2})$. The running time of
the algorithm is less than
$\mathcal{P}(M,((kd)^n d_0)^{m+n})$. For every point
$(\xi_1,\dots,\xi_n)\in\mathcal{J}$ the algorithm constructs an irreducible over
$\mathbb{Q}_m$
polynomial
$\Phi\in\mathbb{Q}_m[Z]$, and the expressions
$\xi_i=\xi_i(\omega)=\sum\limits_{0\leqslant j<\mathrm{deg}(\Phi)}\alpha_j^{(i)}\omega^j\in\mathbb{Q}_m[\omega]$,
where
$\alpha_j^{(i)}\in\mathbb{Q}_m$ (
$1\leqslant i\leqslant n$) and
$\omega\in\widetilde{\mathbb{Q}}_m$,
$\Phi(\omega)=0$. The constructed polynomials and expressions
satisfy the following bounds: $\mathrm{deg}_{\delta_1,\dots,\delta_m,X_1,\dots,X_n}(\Phi)\leqslant d_0\mathcal{P}((kd)^n)$;
$l(\Phi)$, $l(\xi_j(\omega))\leqslant(M+md_0)\mathcal{P}((kd)^n)$. Besides that, in the case
$m=0$, the
algorithm produces a pair of rational numbers
$b_1,b_2\in\mathbb{Q}$ such
that inside the interval
$(b_1,b_2)\subset\mathbb{R}$ there is a unique real root
$\omega\in(b_1,b_2)$ of
$\Phi$, herewith
$l(b_1), l(b_2)\leqslant M\mathcal{P}((kd)^n)$.
UDC:
519.5+512.46