Computing the Monodromy Group of a Plane Algebraic Curve Using a New Numerical-modular Newton-Puiseux Algorithm

Transcription

1 Computing the Monodromy Group of a Plane Algebraic Curve Using a New Numerical-modular Newton-Puiseux Algorithm Poteaux Adrien XLIM-DMI UMR CNRS 6172 Université de Limoges, France SNC'07 University of Western Ontario, Canada

2 Outline 1 Computing the monodromy group of a plane algebraic curve 2 A new numerical-modular Newton-Puiseux algorithm

3 The problem K subeld of C. F K[X, Y ] squarefree and monic in Y. C = {(x, y) C 2 F (x, y) = 0} the associated curve. Fiber at x 0 : F(x 0 ) = {roots of F (x 0, Y ) = 0}. Regular point : #F(x 0 ) = D Y. Critical point : #F(x 0 ) < D Y. δ(x 0 ) : distance between x 0 and its nearest critical point.

4 Regular points Let x 0 regular and F(x 0 ) = {y 1, y 2,..., y DY } the ber at x 0. Implicit function theorem : there exist D Y series Y i (X ) = α ik (X x 0 ) k s.t. F (X, Y i (X )) = 0 in the k=0 neighborhood of x 0 and Y i (x 0 ) = y i. The convergence radius of this series is at least δ(x 0 ). If γ is a path which does not meet any critical point, we can analytically continue the Y i along γ.

5 Critical points : Puiseux Series There exist d series Y ij (X ) = α ik ζe jk i (X x 0 ) k e i s.t. k=0 F (X, Y ij (X )) = 0 for all 1 j e i, 1 i s, with : ζ e = exp ( ) 2 πi e. e 1,..., e s a partition of D Y. The integer e i is the ramication index.

6 Critical points : Puiseux Series There exist d series Y ij (X ) = α ik ζe jk i (X x 0 ) k e i s.t. k=0 F (X, Y ij (X )) = 0 for all 1 j e i, 1 i s, with : ζ e = exp ( ) 2 πi e. e 1,..., e s a partition of D Y. The integer e i is the ramication index. G(X, Y ) = Y 3 X Examples at x 0 = 0 : Y 1 (X ) = X 1 3, Y 2 (X ) = jx 1 3, Y 3 (X ) = j 2 X 1 3. H(X, Y ) = (Y 3 X ) ((Y 1) 2 X ) (Y 2 X 2 ) + X 2 Y 5 Y 1k (X ) = j k X X j k X , k = 1, 2, 3. Y 2k (X ) = 1 + X 1 2 ± 1 2 X X 2 +, k = 1, 2. Y 31 (X ) = 2 3 X X 3 +

7 Local monodromy H(X, Y ) = (Y 3 X ) ((Y 1) 2 X ) (Y 2 X 2 ) + X 2 Y 5 Y 31 (X ) = 2 3 X X 3 + e = 1 : 1-cycle. Y 2k (X ) = 1 + X 1 2 ± 1 2 X e = 2 : 2-cycle. Y 1k (X ) = j k X X j k X e = 3 : 3-cycle. Local monodromy : Permutation generated on the ber. We get it from ramification indices!

8 Global monodromy Let c 1,..., c d denote the critical points. We x a regular base point a. We will compute the d permutations σ 1,..., σ d generated by γ 1,..., γ d. These permutations generate the monodromy group.

9 Motivations Galois theory. Multivariate polynomial factorization (Galligo-Van Hoeij ). First step towards an eective Abel-Jacobi theorem. Integration of algebraic functions (logarithmic part). Applications in Physics (KP equations...). Our long term goal : An implementation of the Abel map with provable accuracy bound.

10 Compute bers and connect 1 Choice of paths. 2 Choice of connection points. a

11 Compute bers and connect 1 Choice of paths. 2 Choice of connection points. a

12 Compute bers and connect 1 Choice of paths. 2 Choice of connection points. 3 Connection method. a

13 Monodromy : state of art (sketch) 1 Compute bers and connect Van Hoeij & Deconinck (2001) Maple's monodromy command. Fibers connected with rst order approximation. Heuristic connection criteria and error control. 2 Dierential equation Chudnovsky & Chudnovsky, Van der Hoeven...

14 Contributions 1 Choice of paths : Euclidean minimum spanning tree. 2 Connection method : truncated series expansions and Puiseux series above critical points. Certied connections. Local monodromy for free. Useful for Abel's map (Deconinck and Patterson 2007). A new numerical-modular algorithm to compute singular part of Puiseux series. 3 Choice of connection points : trade-o between truncation orders and number of steps. Bound for the number of steps.

15 Euclidean Minimum Spanning Tree

16 Connections along the tree

17 Number of connection points At this point, we need O(d) = O(D 2 ) connection points.

18 Truncation orders Let Y (X ) = k=0 µ k(x x 0 ) k e a Puiseux series, Ỹ (X ) = n k=0 µ k(x x 0 ) k e its order n truncation, x 1 D(x 0, ρ) with ρ < δ(x 0 ), M sup Y (x), x D(x 0,ρ) η R + the required precision, ) 1 e β =, ( x1 x 0 δ(x 0 ) Proposition n ln ( η ) M + ln(1 β) 1 Y (x 1 ) Ỹ (x 1 ) η ln(β) With F (X, Y ) = Y 3 X 5 + 2(10X 1) 2, we get β and n

19 Number of connection points If β = 1 2, we only need n 1 log 2 ( η M ). For [c jk, c k ], logarithmic number of connection points : ( ( )) δ(cjk ) O log 3 δ(c k ). Theorem If F Z[X, Y ], we need O(D 6 log F ) connection points to compute the monodromy group. bound cubic in the size of the output.

20 1 Minimize the length of the paths. 2 Series expansions : trade-o between truncation orders and number of steps. Bound for the number of steps. 3 Puiseux series above critical points : a fast numerical-modular algorithm.

21 Newton-Puiseux algorithm : singular part Symbolic algorithm : Bit complexity O(D Y 32 DX 4 ) (Walsh 2000) Purely numerical computation dicult : F (X, Y ) F (X q, Y + ξ 1 q X m ) F (X, Y ) = i, j a ij X j Y i Characteristic polynomial: φ (Z) = i i 0 a ij Z q (i, j) ξ root of φ.

22 A numerical-modular algorithm 1 Compute singular part of Puiseux series modulo a good p. This gives us : Rational exponents m with q 1. q Multiplicities of roots of φ (Z) when it has several factors. Choice of p : can be deduced from the discriminant. 2 Guide numerical computations by this modular information. Complexity for the singular part : O (D Y 4 DX 2 ) modular and oating point computations.

23 Numerical precision F (X, Y ) = (Y 3 M 10,6 (X ))(Y 3 M 10,3 (X )) + Y 2 X 5 A factor of the discriminant has 30 degree and coecients > Number of correct digits for the singular part coecients : Digits Symbolic + Numeric Our algorithm

24 Running time ( Y n 2 P n G n (X, Y ) = with P n0 (X ) = 1 ( n 03! X X n 0 + (n 0 1) X 1 n 0! ) 2 (X ) G n 2 (X, Y ) ). Polynomial Running time Our algorithm for symbolic time precision G s s 9 G s s 9 G s s 9 G s s 9 G s s 9 G s s 9 G s s 9 Prime number used : p =

25 Conclusions Modular computations yield an ecient algorithm for oating point Puiseux series above critical points. Monodromy groups can be computed using controlled number of steps and truncation orders. Paths can be optimized. Future work Control round-o errors and analyze accuracy. Rene the monodromy algorithm. Finalize complexity. Improve implementation.