Research Article Computing Singular Points of Projective ...Remark . We solve univariate polynomials...
Transcript of Research Article Computing Singular Points of Projective ...Remark . We solve univariate polynomials...
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Research ArticleComputing Singular Points of Projective Plane Algebraic Curvesby Homotopy Continuation Methods
Zhongxuan Luo,1,2 Erbao Feng,1 and Jielin Zhang1
1 School of Mathematical Sciences, Dalian University of Technology, Dalian 116024, China2 School of Software, Dalian University of Technology, Dalian 116620, China
Correspondence should be addressed to Zhongxuan Luo; [email protected]
Received 10 April 2014; Accepted 8 May 2014; Published 5 June 2014
Academic Editor: Baodong Zheng
Copyright Β© 2014 Zhongxuan Luo et al.This is an open access article distributed under theCreative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
We present an algorithm that computes the singular points of projective plane algebraic curves and determines their multiplicitiesand characters.The feasibility of the algorithm is analyzed. We prove that the algorithm has the polynomial time complexity on thedegree of the algebraic curve.The algorithm involves the combined applications of homotopy continuation methods and a methodof root computation of univariate polynomials. Numerical experiments show that our algorithm is feasible and efficient.
1. Introduction
Algebraic curves are a classic research object in mathematics.The related computation of algebraic curves arises in sev-eral applications, including number theoretic problems [1],ancient and modern architectural designs, error-correcting[2], biological shape [3], cryptographic algorithms [4, 5], andcomputer aided geometric design [6, 7]. Singular points andtheir multiplicities and characters play an important role inthe research of algebraic curves [8]. They help to determinethe genus of an algebraic curve. Singular points also showsome shape features, such as nodes, self-intersections or cuspsof real curves in robot motion planning, computer aidedgeometry design, andmachine vision. And the determinationof geometric shape and topology of the real curves dependson the singular points. The computation of singular points isalso crucial for tracing curves algorithms [9].
As singular points of an algebraic curve are the solutionsof a polynomial system, most of algorithms in [10β15] solvethe polynomial system either by GroΜbner basis methoddescribed in [16] or by resultant computation. These meth-ods rely on symbolic algebraic computation that requiresexact input of coefficients of algebraic curves. It is easy forthem to suffer from the overwhelming coefficient swell. Sothese methods may be limited to relatively small problem.Furthermore, it is difficult to obtain exact coefficients of
algebraic curves due to data error in many fields of scienceand engineering.
Although we have presented an algorithm to computesingular points of irreducible algebraic curves in [17], thealgorithm is almost experimental and related analysis ofthe algorithm is not provided, for instance, the feasibilityand complexity. The aims of this paper are analyzing thefeasibility of this algorithm for computing singular points ofreducible algebraic curves and proving that the algorithm haspolynomial time complexity on the degree of an algebraiccurve. We also present the effect of tiny perturbations ofcoefficients of algebraic curves on singular points and it maybe a part of reasons why we compute the singular pointsnumerically.
Our algorithm is totally numerical and can deal with alge-braic curves with inexact coefficients. It includes the applica-tions of homotopy continuation methods and the method ofcalculating the multiple roots of univariate polynomials withinexact coefficients without using multiprecision arithmetic.Homotopy continuation method is a reliably and efficientlynumerical method to solve the polynomial systems [18].Several methods have been presented to compute roots ofunivariate polynomials, such as Laguerreβs method, Jenkins-Traub method, and the QR algorithm with the companionmatrix. However, they cannot overcome a barrier of attain-able accuracy on an π multiple root [19, 20]. If we have π
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Hindawi Publishing CorporationDiscrete Dynamics in Nature and SocietyVolume 2014, Article ID 230847, 9 pageshttp://dx.doi.org/10.1155/2014/230847
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digits coefficients accuracy and π2digits machine precision,
the attainable accuracy is min{π1, π2}/π digits. For example,
if the standard double precision of 16 decimal digits is usedand the accuracy of coefficients of the polynomial is 15 digits,only 3 correct digits of a root of multiplicity 5 can be obtainedby the above methods. A method proposed by Zeng [21] isnot subject to the accuracy barrier and a lot of numericalexperiments have shown its efficiency and robustness.
The following section presents the basic notions onalgebraic curves and singular points. In Section 3, we focuson the algorithm of computing singular points. Section 4is devoted to the numerical experiments. We conclude thispaper in Section 5.
2. Preliminaries
In this section, we introduce some basic notions and resultson algebraic curves and singular points. LetC be the complexnumbers field and let P2(C) be the projective plane over C.
Definition 1 (see [8, 22]). A projective plane algebraic curveover C is defined as the set
C = {(π : π : π) β P2(C) | πΉ (π, π, π) = 0} (1)
for a nonconstant square-free homogeneous polynomialπΉ(π₯, π¦, π§) β C[π₯, π¦, π§].
We call πΉ(π₯, π¦, π§) the defining polynomial of C. Thedegree of polynomial πΉ(π₯, π¦, π§) is called the degree ofC.
We take the line π§ = 0 in P2(C) as the line at infinity.If the projective plane algebraic curve C is defined by thepolynomial πΉ(π₯, π¦, π§), then the corresponding affine planealgebraic curve Cβ is defined by the dehomogenizationπ(π₯, π¦) of πΉ(π₯, π¦, π§),
Cβ= {(π, π) β A
2(C) | π (π, π) = 0} , (2)
where A2(C) is the affine plane over C.Therefore, if
πΉ (π₯, π¦, π§) = ππ(π₯, π¦) + π
πβ1(π₯, π¦) π§ + β β β + π
0(π₯, π¦) π§
π,
(3)
then
π (π₯, π¦) = ππ(π₯, π¦) + π
πβ1(π₯, π¦) + β β β + π
0(π₯, π¦) , (4)
where ππ(π₯, π¦) is a homogeneous polynomial of degree π and
ππ(π₯, π¦) is nonzero.Throughout this paper, whenever we speak of a βcurveβ
we mean an βalgebraic curve.β For brevity, we will usuallyuse the phrase βthe curve πΉ(π₯, π¦, π§) = 0β instead of βtheprojective plane algebraic curvewhose defining polynomial isπΉ(π₯, π¦, π§).β Of course, a polynomialπΊ = ππΉ, for some nonzeroπ β C, defines the same curve, so πΉ is unique only up tomultiplication by nonzero constants. The above usage is alsofor the case of affine plane algebraic curve.
Every point (π, π) on Cβ corresponds to a point (π : π :1) onC, and every additional point onC is a point at infinity.
In other words, the first two coordinates of additional pointsare the nontrivial solutions of π
π(π₯, π¦) = 0, with the third
coordinates being 0.Thus, the curveC has only finitely manypoints at infinity.
Definition 2 (see [8, 22]). LetCβ be an affine plane algebraiccurve overCdefined byπ(π₯, π¦) β C[π₯, π¦], and letπ = (π, π) βCβ. π is of multiplicity π on Cβ if and only if all derivativesof π(π₯, π¦) up to and including the (π β 1)th vanish at π, but atleast one πth derivative does not vanish at π.
A point of multiplicity two ormore is said to be a singularpoint, and especially the point of multiplicity two is called adouble point. A point of multiplicity one is called a simplepoint.
It is evident that a necessary and sufficient condition thata point (π, π) of the curve π(π₯, π¦) = 0 is singular is that
π (π, π) = 0,
ππ₯(π, π) = 0,
ππ¦(π, π) = 0.
(5)
Definition 3 (see [8]). Let the parametric equations of tan-gents to the curve π(π₯, π¦) = 0 at π = (π, π) be
π₯ = π + ππ‘,
π¦ = π + ππ‘,
(6)
and the tangents to the curve π(π₯, π¦) = 0 at π = (π, π) ofmultiplicity π are determined by the ratioπ :π and correspondto the roots of
ππ₯πππ+ (
π
1)ππ₯πβ1π¦ππβ1π + β β β + (
π
π)ππ¦πππ= 0, (7)
where ππ₯πβππ¦π = (π
ππ/ππ₯πβπππ¦π)(π, π), and they are counted
with multiplicities equal to the multiplicities of the corre-sponding roots of this equation.
Definition 4 (see [8, 22]). A singular point π of multiplicity πon an affine plane algebraic curveCβ is called ordinary if andonly if the π tangents toCβ at π are distinct and nonordinaryotherwise.The property of singular point π of being ordinaryor nonordinary is called the character of π.
The criteria for themultiplicity of a point on the projectiveplane algebraic curve can be characterized as follows.
Proposition 5 (see [8]). π β P2(C) is a point of multiplicityπ on the projective plane algebraic curve πΉ(π₯, π¦, π§) = 0 if andonly if all the (π β 1)th derivatives of πΉ(π₯, π¦, π§), but not all theπth derivatives, vanish at π.
As a corollary of this proposition, a point π of the curveπΉ(π₯, π¦, π§) = 0 being singular can be put in a more convenientform.
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Proposition 6 (see [8, 22]). π β P2(C) is a singular point ofthe projective plane algebraic curve πΉ(π₯, π¦, π§) = 0 if and only if
ππΉ
ππ₯
(π) =
ππΉ
ππ¦
(π) =
ππΉ
ππ§
(π) = 0. (8)
The following two theorems are of use for analyzing thealgorithm of the next section.
Theorem 7 (see [8]). If a curve of degree π with no multiplecomponents has multiplicities π
πat points π
π, then
(π β 1) (π β 2) β₯ βππ(ππβ 1) . (9)
Theorem 8 (see [8] BeΜzoutβs theorem). Two curves of degreesπ and π with no common components have exactly ππintersections.
3. Computation of Singular Points
In this section, a method of solving the overdeterminedpolynomial systems is presented first. We also outline thealgorithm on computing the singular points of projectiveplane algebraic curves, and afterwards we analyze feasibilityand complexity of the algorithm.The last subsection includesthe effect of tiny perturbations of coefficients of the curve onsingular points.
3.1. Solving Overdetermined Polynomial Systems. The follow-ing proposition shows how to reduce an overdeterminedpolynomial system to a square system that most of the papersand software programs on homotopy continuation methodfocus on. Let π and π be the number of equations andunknowns, respectively, and π > π.
Proposition9 (see [23, 24]). There are nonempty Zariski opendense sets of parameters π
ππβ Cπ(πβπ) or π
ππβ Rπ(πβπ) such
that every isolated solution of
π1(π₯1, . . . , π₯
π) = 0,
...ππ(π₯1, . . . , π₯
π) = 0
(10)
is an isolated solution of
π1(π₯1, . . . , π₯
π) +
π
β
π=π+1
π1πππ(π₯1, . . . , π₯
π) = 0,
...
ππ(π₯1, . . . , π₯
π) +
π
β
π=π+1
πππππ(π₯1, . . . , π₯
π) = 0.
(11)
The solutions set of an overdetermined system belongs tothat of the corresponding square system, but the converse isnot true.
3.2. Algorithm. We summarize the algorithm described in[17] for computing the singular points of irreducible projec-tive plane algebraic curves.
Algorithm 1.
Input. Consider projective plane algebraic curve πΉ(π₯, π¦, π§) =0 and corresponding affine plane algebraic curve π(π₯, π¦) = 0.Threshold is π > 0.
Output. Consider singular points and their multiplicities andcharacters of the curve πΉ(π₯, π¦, π§) = 0.
Step 1 (randomization). By Proposition 9 and choosing therandom complex numbers πΌ and π½, we add randommultiplesof the last equation to the first two equations of the polyno-mial system (5):
π (π₯, π¦) + πΌππ¦(π₯, π¦) = 0,
ππ₯(π₯, π¦) + π½π
π¦(π₯, π¦) = 0.
(12)
Step 2 (computing the singular points (π : π : 1)). Wesolve the polynomial system (12) by the polyhedral homotopycontinuation method first and denote the solutions set as{(π : π : 1)}. If |π(π, π)| < π, (π : π : 1) will be a singularpoint.
Step 3 (computing the singular points (π : π : 0) at infin-ity). Since π
π(π₯, π¦) is a bivariate homogeneous polyno-
mial of degree π, we obtain the points set {(π : π : 0)} atinfinity by solving the corresponding univariate polyno-mial. By Proposition 6, if Max{(ππΉ/ππ₯)(π, π, 0), (ππΉ/ππ¦)(π, π, 0), (ππΉ/ππ§)(π, π, 0)} < π, then (π : π : 0) will bea singular point at infinity.
When we determined the multiplicity and character ofsingular point (π : π : 0) at infinity, (π : π : 0) willbe transferred to point (π : π : 1) by simple lineartransformation.
Step 4 (determining the multiplicity). Evaluating the deriva-tives of π(π₯, π¦) at a singular point from order equal to 2, ifthe moduli of all derivatives of π(π₯, π¦) up to and includingthe (π β 1)th are less than π, but at least one modulus ofπth derivatives is greater than π, then the multiplicity of thissingular point will be determined as π.
Step 5 (determining the character). The tangents to the curveπ(π₯, π¦) = 0 at a singular point of multiplicity π correspondto the roots of (7). Whether the singular point is ordinary ornot will be determined if there are multiple roots of (7).
3.3. Remarks on Feasibility of Algorithm. The following theo-rems and remarks will show the feasibility of Algorithm 1 forcomputing the singular points of reducible projective planealgebraic curves.
Theorem 10. All the solutions of polynomial system (12) areisolated.
Proof. Since π(π₯, π¦) is square-free, π(π₯, π¦) and ππ₯(π₯, π¦)
have no nonconstant common divisors. It also holds forπ(π₯, π¦) and π
π¦(π₯, π¦). In Step 1 of Algorithm 1, for the ran-
dom complex numbers πΌ and π½, π(π₯, π¦) + πΌππ¦(π₯, π¦) and
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ππ₯(π₯, π¦) + π½π
π¦(π₯, π¦) also have no nonconstant common
divisors. It follows that all the solutions of polynomial system(12) are isolated.
Remark 11. The inequality in Theorem 7 shows that anirreducible curve has finitely many singular points. So, thesingular point of an irreducible curve is isolated.
Theorem 12. A reducible curve π(π₯, π¦) = 0 has only finitelymany singular points and they are all isolated points.
Proof. Without loss of generality, we may assume thatπ(π₯, π¦) = π
1(π₯, π¦)π
2(π₯, π¦), where π
1(π₯, π¦) and π
2(π₯, π¦) are
irreducible polynomials. Obviously, the singular points ofπ(π₯, π¦) = 0 are the singular points of π
π(π₯, π¦) = 0 (π =
1, 2) and the intersections of π1(π₯, π¦) = 0 and π
2(π₯, π¦) =
0. By Remark 11 and Theorem 8, the set of singular pointsof π(π₯, π¦) = 0 is finite and the singular points are allisolated.
Remark 13. When solving the polynomial system (12) inStep 2 of Algorithm 1 by homotopy continuation methods,Proposition 9 and Theorem 12 imply that all singular points(π : π : 1) of reducible curve can be determined.
Remark 14. In Step 2 of Algorithm 1, the Jacobian of polyno-mial system (12) is
(
ππ₯+ πΌππ₯π¦
ππ¦+ πΌππ¦π¦
ππ₯π₯+ π½ππ₯π¦ππ₯π¦+ π½ππ¦π¦
) . (13)
Thismatrix is singular at a singular point of multiplicity threeat least. The numerical techniques of homotopy continuationmethods could deal with this well.
Remark 15. As π(π₯, π¦) is a nonzero polynomial and has finitedegree π, the modulus of some derivative of order less than orequal to πmust be greater than the given threshold at a singu-lar point. Hence, Step 4 of Algorithm 1 ends for determiningthe multiplicities of finitely many singular points.
Remark 16. We solve univariate polynomials in Steps 3and 5 of Algorithm 1. Numerical singular points make thecoefficients of univariate polynomials in Step 5 inexact. Wehave mentioned the efficiency of computation of multipleroots of inexact univariate polynomials proposed by Zeng inSection 1. Employing Zengβs method, we compute the singu-lar points at infinity accurately in Step 3 and the characters ofall singular points can be determined correctly in Step 5.
3.4. Computational Complexity. In this subsection, we ana-lyze the computational complexity of Algorithm 1.Thedegreeof the curve π(π₯, π¦) = 0 is denoted by π.
Theorem 17. The complexity of Step 2 in Algorithm 1 isπ(π2).
Proof. Shub and Smale [25, 26] present that finding anapproximate zero of a polynomial system by homotopycontinuation methods can be solved in polynomial time
on the average and the number of arithmetic operations isbounded by ππ4, where π is a universal constant and π isthe number of variables. By BeΜzoutβs Theorem 8, the numberof solutions of polynomial system (12) is π(π β 1) at most.We deduce that the complexity of Step 2 is no more than16ππ(π β 1).
Theorem 18. The complexity of Steps 3 and 5 in Algorithm 1 isπ(π3) and π(π5) or less, respectively.
Proof. Pan reports [19] that the complexity of general root-finders of univariate polynomial of degree π is π(π2) or less,but Zengβs method [21] reaches high accuracy on multipleroots at the higher computing cost ofπ(π3). It follows that thecomplexity of Step 3 is π(π3). From Steps 2 and 3, we knowthat the number of singular points of the curve π(π₯, π¦) = 0of degree π is π2 at most. Therefore, we conclude that thecomplexity of Step 5 is π(π5) or less.
Theorem 19. The complexity of Step 4 in Algorithm 1 does notexceed π(π5).
Proof. The number of all the nonzero derivatives of bivariatepolynomial π(π₯, π¦) of degree π is not more than π(π +3)/2. The complexity of evaluation of a polynomial at apoint is linear in the degree of the polynomial and thenumber of variables. Since there are π2 points at most inStep 4 of Algorithm 1 and the complexity of linear changeof coordinates is polynomial in the number of variables, thecomplexity of Step 4 does not exceed π(π5).
The following theorem holds at once.
Theorem 20. The complexity of Algorithm 1 is polynomialtime in the degree π of the projective plane algebraic curve andis π(π5).
3.5. Effect of Tiny Perturbations on Singular Points. Theauthors [27] conclude that random perturbations of coeffi-cients of plane algebraic curves will almost invariably destroyall singular points. The current methods dealing with planealgebraic curves are almost ill-conditioned with respect totiny perturbations. For example, let us consider the curveπ₯3β π¦2= 0 with nonordinary double singular point (0, 0)
and its perturbation π₯3 β π¦2 + ππ₯2 = 0, where π > 0. It isnatural to compute the approximate singular point out andthe original character for relative magnitude π > 0. However,the character of singular point (0, 0) is changed by usingsymbolic algebraic computation, even if π > 0 is sufficientlysmall.
For the curve π₯3 β π¦2 + ππ₯2 = 0, when π = 0.0000001,the numerical procedure based on Algorithm 1 outputs anonordinary double singular point
(β0.00000006666667 + 0.00000000000000π,
0.00000000000000 β 0.00000000000000π) .
(14)
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4. Numerical Experiments
In this section, Algorithm 1 is implemented in Matlab andsome examples are presented to show the efficiency ofcorresponding numerical procedure. There are many freelyavailable homotopy continuation packages. As Lee et al. [28]report that HOM4PS-2.0 is generally the fastest packagefor solving small to moderately large sparse systems, it isemployed to solve polynomial system (12) in Algorithm 1.Many numerical examples are provided in [17] where thecurves are irreducible and the coefficients of the curves areinexact. We finish this section with two reducible curves.
Example 1. Consider
π (π₯, π¦) = π1(π₯, π¦) π
2(π₯, π¦)
= π₯5π¦2+ π₯4π¦ + π₯6+ π₯6π¦ + 2π₯
4π¦2
+ π₯5+ π₯5π¦ + π₯3π¦4+ π₯4π¦3+ π₯2π¦4
+ π₯3π¦2+ π₯3π¦3β π₯π¦4β π¦3
β π₯2π¦2β π¦4β π₯π¦2β π¦3π₯,
π1(π₯, π¦) = π¦
2π₯ + π¦ + π₯
2+ π¦π₯2+ π¦2+ π₯ + π¦π₯,
π2(π₯, π¦) = π₯
4+ π₯2π¦2β π¦2.
(15)
There are two components π1(π₯, π¦) = 0 and π
2(π₯, π¦) = 0
for the curve π(π₯, π¦) = 0. The curve π1(π₯, π¦) = 0 has no
singular points and its real part is plotted in Figure 1. Thecurve π
2(π₯, π¦) = 0 has nonordinary double singular point
corresponding to (0 : 0 : 1) and ordinary double singular pointcorresponding to (0 : 1 : 0). Figure 2 shows real part of thecurve π
2(π₯, π¦) = 0 and the singular point corresponding to
(0 : 0 : 1) is marked by a solid square.The curves π
1(π₯, π¦) = 0 and π
2(π₯, π¦) = 0 have 8
intersections corresponding toππ(π = 1, . . . , 8).Their related
projective curves intersect π9= (0, 1, 0) at infinity. (0, 0) is
the simple point of π1(π₯, π¦) = 0, nonordinary double point of
π2(π₯, π¦) = 0. Therefore, (0, 0) corresponding toπ
1= (0 : 0 : 1)
is nonordinary singular point of multiplicity of 3 of π(π₯, π¦) =0. π9= (0 : 1 : 0) is ordinary double point and simple point
of the corresponding projective curves for π2(π₯, π¦) = 0 and
π1(π₯, π¦) = 0, respectively. The tangent to the latter at π
9
coincides with one tangent to the former at π9. So, π
9=
(0 : 1 : 0) is nonordinary singular point of multiplicity of 3 ofthe projective curve corresponding to π(π₯, π¦) = 0.
Table 1 lists the results of our numerical procedure for theprojective curve corresponding to π(π₯, π¦) = 0 in Example 1.Figure 3 presents the real part of π(π₯, π¦) = 0 in Example 1.The solid dot marks the real intersection of π
1(π₯, π¦) = 0 and
π2(π₯, π¦) = 0 corresponding to π
5. The real singular point
corresponding to π1= (0 : 0 : 1) is still marked by a solid
square.We use the abbreviations βMult.β and βOrd.β for the words
βmultiplicityβ and βordinaryβ in our tables, respectively.
Example 2. Consider
π (π₯, π¦) = π1(π₯, π¦) π
2(π₯, π¦)
= 3264925π₯4π¦ + 445317π₯
2π¦2+ 1485786π₯
2π¦
+ 1055617π₯4+ 443586π₯
2β 1606976π₯π¦
4
β 222040π¦4β 326949π₯
5π¦ + 2949048π₯π¦
5
+ 421959π₯3π¦3β 295824π¦
2β 283584π¦
3
β 291435π₯2π¦3+ 8419454π₯
3π¦2β 3938342π¦
5π₯2
β 2629937π₯5π¦2+ 270492π₯
9π¦2β 180328π₯
8π¦
β 707315π₯7π¦ + 121188π₯
9π¦ + 2832768π¦
6π₯
+ 572040π¦5π₯5+ 160152π¦
9π₯ + 621050π¦
7π₯3
β 536400π¦4π₯4+ 2513654π₯
3π¦ β 1676336π₯π¦
3
β 3734596π¦6π₯5+ 154032π¦
10π₯ β 102688π¦
9
+ 24438π¦2π₯4+ 491170π₯
7π¦3β 542892π₯
6π¦2
β 1330879π₯7π¦2β 734296π¦
8π₯3β 4707928π¦
5π₯4
+ 215696π¦7π₯2+ 8655317π¦
6π₯3β 1322704π¦
8π₯
β 211840π¦5+ 659312π¦
7β 106768π¦
8
+ 685832π¦6+ 296494π₯
6β 80792π₯
8
+ 496542π₯6π¦ β 698748π¦
6π₯2β 2167689π¦
5π₯3
β 1375744π¦7π₯ + 1350780π¦
4π₯2+ 9826722π¦
4π₯5
β 6460272π¦3π₯4β 18749449π¦
4π₯3
β 346096π₯7π¦4β 250144π₯
6π¦3β 1006320π¦
3π₯5,
π1(π₯, π¦) = 51344π¦
5+ 53384π¦
4β 47264π¦
3
β 415912π₯2π¦3β 49304π¦
2+ 29070π₯
2π¦2
+ 247631π₯2π¦ + 90164π₯
4π¦
+ 73931π₯2+ 40396π₯
4,
π2(π₯, π¦) = 3π₯
5π¦ + 3π₯π¦
5+ 10π₯
3π¦3β 2π₯4
β 2π¦4β 12π₯
2π¦2β 23π₯
3π¦ β 23π₯π¦
3
+ 11π₯2+ 11π¦
2+ 34π₯π¦ + 6.
(16)
The curve π(π₯, π¦) = 0 has two components π1(π₯, π¦) = 0
and π2(π₯, π¦) = 0. There are six singular points corresponding
to ππ(π = 1, . . . , 6) for the curve π
1(π₯, π¦) = 0 and they
are marked by solid squares in Figure 4 where the real partof the curve π
1(π₯, π¦) = 0 is plotted. Four singular points
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Table 1: Results for π(π₯, π¦) = 0 in Example 1.
π₯ π¦ π§ Mult. Ord.π1
0.00000003 + 0.00000005π 0.00000000 + 0.00000000π 1.00000000 3 Noπ2
β0.89868027 + 0.41873939π β0.15046254 + 1.06341745π 1.00000000 2 Yesπ3
0.36804326 β 1.20320368π β0.92239819 β 0.40178011π 1.00000000 2 Yesπ4
0.36804326 + 1.20320368π β0.92239819 + 0.40178011π 1.00000000 2 Yesπ5
β0.49367385 β 0.00000000π 0.28024456 + 0.00000000π 1.00000000 2 Yesπ6
β0.89868027 β 0.41873939π β0.15046254 β 1.06341745π 1.00000000 2 Yesπ7
0.77747393 + 0.21532054π β0.56726154 β 0.66500198π 1.00000000 2 Yesπ8
0.77747393 β 0.21532054π β0.56726154 + 0.66500198π 1.00000000 2 Yesπ9
0.00000000 1.00000000 0 3 No
0
1
β1
β2
β3
β4
β5
0 1β1β2β3β4β5
Figure 1: Real part of π1(π₯, π¦) = 0 in Example 1.
corresponding to ππ(π = 7, . . . , 10) of the curve π
2(π₯, π¦) = 0
are illustrated by the solid triangles in Figure 5 where thereal part of the curve π
2(π₯, π¦) = 0 is presented. Obviously,
ππ(π = 1, . . . , 10) are singular points of the projective curve
corresponding to the curve π(π₯, π¦) = 0.There are 29 intersections corresponding to π
π(π =
11, . . . , 39) for the curves π1(π₯, π¦) = 0 and π
2(π₯, π¦) = 0.
Their related projective curves intersect π40at infinity. These
30 intersections ππ(π = 11, . . . , 40) are also singular points of
the projective curve corresponding to the curve π(π₯, π¦) = 0.So the curve π(π₯, π¦) = 0 has 40 singular points.
Figure 6 shows the real part of the curve π(π₯, π¦) = 0whose real singular points are marked by the solid dots,squares, and triangles. The solid dots mark the real inter-sections of the curves π
1(π₯, π¦) = 0 and π
2(π₯, π¦) = 0. The
solid squares and triangles illustrate the singular points of thecurves π
1(π₯, π¦) = 0 and π
2(π₯, π¦) = 0, respectively. Table 2
lists the results of our numerical procedure for the projectivecurve corresponding to π(π₯, π¦) = 0 in Example 2.
It takes 0.28628 seconds and 1.26786 seconds for ournumerical procedure to obtain the results of Examples 1 and2, respectively, on the Lenovo PC with Pentium Dual Core,
0.5 1.5β0.5β1.5
0
1
β1
β2
2
0.0β1.0 1.0
Figure 2: Real part of π2(π₯, π¦) = 0 in Example 1.
0
1
β1
β2
β3
2
0 1β1β2β3 2
Figure 3: Real part of π(π₯, π¦) = 0 in Example 1.
-
Discrete Dynamics in Nature and Society 7
Table 2: Results for π(π₯, π¦) = 0 in Example 2.
π₯ π¦ π§ Mult. Ord.π1
0.00000000 0.00000000 β 0.00000000π 1.00000000 2 Yesπ2
β0.50000000 β 0.00000000π 0.99999999 + 0.00000000π 1.00000000 2 Yesπ3
β0.00000000 β 0.00000000π β1.00000000 + 0.00000000π 1.00000000 2 Yesπ4
0.99999999 + 0.00000000π β0.50000000 β 0.00000000π 1.00000000 2 Yesπ5
β1.00000000 + 0.00000000π β0.50000000 + 0.00000000π 1.00000000 2 Yesπ6
0.50000000 + 0.00000000π 1.00000000 + 0.00000000π 1.00000000 2 Yesπ7
2.41421356 β 0.00000000π 0.41421356 β 0.00000000π 1.00000000 2 Yesπ8
β2.41421356 + 0.00000000π β0.41421356 β 0.00000000π 1.00000000 2 Yesπ9
0.41421356 β 0.00000000π 2.41421356 β 0.00000000π 1.00000000 2 Yesπ10
β0.41421356 β 0.00000000π β2.41421356 + 0.00000000π 1.00000000 2 Yesπ11
1.86368244 + 0.00000000π β0.81167781 + 0.00000000π 1.00000000 2 Yesπ12
β0.56599742 + 0.66867689π 0.95095432 β 0.12549545π 1.00000000 2 Yesπ13
β0.58308154 + 0.00000000π β2.34385106 + 0.00000000π 1.00000000 2 Yesπ14
1.67570344 + 0.00000000π 1.32326562 + 0.00000000π 1.00000000 2 Yesπ15
0.79829233 1.77539598 + 0.00000000π 1.00000000 2 Yesπ16
0.09549542 β 0.39009016π β0.97196258 + 0.89462218π 1.00000000 2 Yesπ17
0.52445070 + 0.00000000π β2.19040368 β 0.00000000π 1.00000000 2 Yesπ18
0.05975897 β 0.25434442π β0.06366391 β 0.36672723π 1.00000000 2 Yesπ19
0.15502368 β 1.39689139π β0.01303583 + 0.19325672π 1.00000000 2 Yesπ20
0.51269502 + 0.43085324π β0.51116222 + 0.08768075π 1.00000000 2 Yesπ21
0.15502368 + 1.39689139π β0.01303583 β 0.19325672π 1.00000000 2 Yesπ22
0.09549542 + 0.39009016π β0.97196258 β 0.89462218π 1.00000000 2 Yesπ23
0.54660137 + 0.59723958π β0.36484540 β 0.17346068π 1.00000000 2 Yesπ24
1.40455064 + 0.00000000π 1.22984070 + 0.00000000π 1.00000000 2 Yesπ25
β2.11390876 β 0.00000000π β0.91044961 β 0.00000000π 1.00000000 2 Yesπ26
β2.39860964 + 0.00000000π β0.45929305 β 0.00000000π 1.00000000 2 Yesπ27
β0.35998047 + 0.25179313π 0.61198862 β 0.60194230π 1.00000000 2 Yesπ28
β1.29601698 β 0.00000000π 1.19480441 + 0.00000000π 1.00000000 2 Yesπ29
0.05975897 + 0.25434442π β0.06366391 + 0.36672723π 1.00000000 2 Yesπ30
β0.51418694 β 0.00000000π β2.16372657 β 0.00000000π 1.00000000 2 Yesπ31
β1.79027097 + 0.00000000π β0.78332475 β 0.00000000π 1.00000000 2 Yesπ32
β0.82449210 + 0.00000000π 1.84542750 + 0.00000000π 1.00000000 2 Yesπ33
β0.35998047 β 0.25179313π 0.61198862 + 0.60194230π 1.00000000 2 Yesπ34
β2.28499755 β 0.00000000π β0.46036727 + 0.00000000π 1.00000000 2 Yesπ35
2.24284114 β 0.00000000π β0.46080446 β 0.00000000π 1.00000000 2 Yesπ36
0.92084852 + 0.00000000π 2.10459508 + 0.00000000π 1.00000000 2 Yesπ37
β0.56599742 β 0.66867689π 0.95095432 + 0.12549545π 1.00000000 2 Yesπ38
0.54660137 β 0.59723958π β0.36484540 + 0.17346068π 1.00000000 2 Yesπ39
0.51269502 β 0.43085324π β0.51116222 β 0.08768075π 1.00000000 2 Yesπ40
1.00000000 0.00000000 0.00000000 2 Yes
CPU of 2.5GHZ, and memory of 1.99GB. We usually choose10β6 as the thresholds in Steps 2, 3, and 4 in Algorithm 1.
5. Conclusions
This paper provides an effective algorithm for computingthe singular points of projective plane algebraic curves by
homotopy continuationmethods.The determination of mul-tiplicities relies on the accuracy of singular points. The char-acters of singular points are determined by Zengβs method forcomputing multiplicities of roots of inexact univariate poly-nomials. The precision of coefficients of inexact univariatepolynomials in our algorithm depends on the accuracy ofsingular points. Several numerical examples are presented to
-
8 Discrete Dynamics in Nature and Society
0
1
β1
β2
2
0 1β1β2 2
Figure 4: Real part of π1(π₯, π¦) = 0 in Example 2.
0
1
β1
β2
β3
2
3
0 1β1β2β3 2 3
Figure 5: Real part of π2(π₯, π¦) = 0 in Example 2.
illustrate the efficiency of our algorithm. We will discuss theadaptive thresholds of the algorithm in the future.
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper.
Acknowledgment
This paper is supported by the National Natural ScienceFoundation of China (no. 11171052).
0
1
β1
β2
β3
2
3
0 1β1β2β3 2 3
Figure 6: Real part of π(π₯, π¦) = 0 in Example 2.
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