Cyclic n-roots
ì ï ï ï í ï ï ï î |
x1+x2+···+xn = 0 |
x1x2+ x2 x3 + ···+xnx1 = 0 |
· · · |
x1··· xn-1+ x2··· xn+···+xnx1··· xn-2 =
0 |
x1x2··· xn = 1 |
|
A simplified formulation is obtained in terms of n-1 equations
in n-1 new variables by
dividing the k-th polynomial by xnk for k=1,..., n.
The new variables are yi=xi/xn, i=1, ..., n-1, and
the system:
|
y1+y2+··· + yn-1 + 1 |
= |
0, |
|
|
y1 y2+ y2 y3 + ···+ y1 |
= |
0, |
|
|
· · · |
|
y1··· yn-1+ y2··· yn-1+···+ y1··· yn-2 |
= |
0 |
|
|
y1 y2··· yn-1 |
= |
xn-n.
|
|
References for this problem can be found in [2].
Characteristics:
- Dimension 0 for n=1,2,3,5,6,7.
- Positive-dimensional variety for n=4,8.
- Degree known for n£ 8.
n |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
degree |
1 |
2 |
6 |
16 |
70 |
156 |
924 |
1152 |
Degree £ (
)
(see [5]).
Example 1:
proc(n)
local i,j,k,kk,polys,factor;
polys := array(1..n);
for i from 1 to n-1 do # poly
polys[i] := 0;
for j from 1 to n do # start var
factor := 1;
for k from j to j+i-1 do
if k <= n then factor := factor * x[k];
else kk := k-n; factor := factor * x[kk]; fi;
od;
polys[i] := polys[i] + factor;
od;
od;
polys[n] := 1;
for i from 1 to n do polys[n] := polys[n] * x[i]; od;
polys[n] := polys[n] - 1;
convert(polys,list);
end:
Problem: Bound the number of roots.
· Solution by I.Z Emeris, INRIA, Projet SAGA,
2004 Route des Lucioles,BP 93, 06902 Sophia-Antipolis (France),
emeris@sophia.inria.fr, (5 May 1995)
Mixed volumes bound the number of isolated roots.
The results concern the Lift-Prune algorithm by
Emiris and Canny (1995) [4].
Its public implementation is accessible at
ftp://www.inria.fr/saga/emiris.
|
#isolated |
mixed |
Lift-Prune |
n |
roots |
volume |
CPU time |
3 |
6 |
6 |
0s |
4 |
0 |
16 |
0s |
5 |
70 |
70 |
0s |
6 |
156 |
156 |
2s |
7 |
924 |
924 |
27s |
8 |
1152 |
2560 |
4m 19s = 259s |
9 |
unknown |
11016 |
40m 59s = 2459s |
10 |
unknown |
35940 |
4h 50m 14s = 17414s |
11 |
unknown |
184756 |
38h 26m 44s = 138404s |
Table 1: Lift-Prune executed on a DEC Alpha 3300.
Problem: Compute a resultant matrix.
· Solution by I.Z Emeris, INRIA, Projet SAFIR, 2004 Route des Lucioles,BP
93, 06902 Sophia-Antipolis (France),
emeris@sophia.inria.fr, 5 May 1995
Sparse resultants essentially produce multiplication tables and
reduce the solution of zero-dimensional problems to an eigencomputation.
Some preliminaty results on a SUN Sparc 20 follow for the
simplified formulation.
deg R stands for the total degree of the sparse resultant
in the input coefficients, M is the resultant matrix computed
by the incremental algorithm [4], column ``greedy''
gives the dimension of the resultant matrix constructed by the greedy
algorithm [3].
n |
deg R |
dim M |
rank M |
greedy |
time |
4 |
6 |
6 |
5 |
7 |
0s |
5 |
20 |
26 |
26 |
32 |
0s |
6 |
66 |
102 |
102 |
176 |
3s |
7 |
270 |
685 |
627 |
³ 1273 |
1508s |
Table 2: The incremental and greedy algorithms.
References
- [1]
-
D. Bini and B. Mourrain.
Polynomial test suite.
1996.
- [2]
-
G. Björck and R. Fröberg.
A faster way to count the solutions of inhomogeneous systems of
algebraic equations, with applications to cyclic n-roots.
J. Symbolic Computation, 12:329--336, 1991.
- [3]
-
J. Canny and P. Pedersen.
An algorithm for the Newton resultant.
Technical Report 1394, Comp. Science Dept., Cornell University,
1993.
- [4]
-
I.Z. Emiris and J.F. Canny.
Efficient incremental algorithms for the sparse resultant and the
mixed volume.
J. Symbolic Computation, 20(2):117--149, August 1995.
- [5]
-
L. Pottier.
Bounds for degree of the n-cyclic system.
INRIA Sophia-Antipolis, Manuscript, 1995.