LIST OF PUBLICATIONS

Current work in progress (to be submitted in 2008)

1. Speckle Tracking on Cardiac Echographic image sequences
R. Azencott, D. Khoury MD, Y.  Li   
(to be submitted to Trans. IEEE Signal Processing)  2008

2. Modeling Genes Transcription Networks from high-throughput data
R. Azencott, A. Cooney, P. Gunaratne, Z. Luo (in preparation)  2008

3. Parametric estimation for systems of stochastic differential equations
R. Azencott, Y. Ghadyan, R. Glowinski (in preparation)  2008

4. Exact and approximate Maximum Likelihood estimates for parameters of
     multidimensional gaussian  diffusion processes
R. Azencott, A. Beri, I. Timofeyev (in preparation)  2008

5. Suport Vector Machines  and predictors of traffic injury level 
    R. Azencott, M. Mougeot
(to be submitted to “Traffic Injury Prevention” ) 2008

6. Rotation invariant multi-resolution analysis of 3D images and texture identification
R. Azencott, S. Alexander, , S.Jain, E. Papadakis (in preparation) 2008

7. Proteomic Mass Spectra : optimal parameters for MALDI data aquisition
R. Azencott, S. Alexander, A. Bouamrani, E. Tasciotti, M. Ferrari (in preparation)  2008

8. (Book)  Automatic learning , information theory, and pattern recognition         
R. Azencott, J. Lacaille, C. Lehalle, M. Mougeot, J. Wang         
140 pages  ( in finalization stage, to be published by Springer) 2008

 

Patents (inventor R. Azencott)

Patent  1  : A method for anomaly detection
Non supervised algorithmics for automatic learning of critical features and anomaly detection in signal analysis
Patent delivered : Europe N° 00 410 009 5 ; USA N° 09 519 597, 2003

Patent  2 :  A method to monitor process performance indicators
Auto-adaptive rigorous probabilistic computation of global performance score, based on simultaneous analysis of large heterogeneous families of dynamic performance indicators.
Patent delivered : Europe N° 00 410 069 9 ; USA N° 09 784 134, 2003

Patent  3 :  A system for automatic real time identification of specific sounds
Automatic live audio-surveillance systems for urban buses, trains, airplanes, etc. implying on board microphones and associated hardware plus sound identification software, auto-calibrated by off line automatic learning.
Patent delivered : Europe N° 03 054 14, 2003

Patent  4  :  A technique for geometric shapes identification on curves
Automatic on line analysis of sensor recorded signals to detect occurences of specific families of fuzzy geometric shapes; self-calibration by autmatic learning.
Patent delivered : Europe N° 00 410 036.8 ; USA N° 09 616 055, 2003

Patent  5  :  A system for content driven navigation in multimedia databases
Automatic content indexation algorithms and softwares for large databases of  images and texts; automated  iterative browsing among documents with similar contents.
Patent delivered  : Europe N° 00 410 103.6 ; USA N°  09 783 621, 2002

 

Published articles, books, scientific softwares

1.  (Book)   Elements of Algebraic Geometry   
R. Azencott, P. Berthelot
based on F. Bruhat  graduate course, 120 pages  Publications ENS Ulm Paris, 1966

2.  Poisson spaces of semi-simple Lie groups
Comptes Rendus Acad. Sciences, Paris, France vol 266 serA pp 970-973, 1968

3.   Poisson spaces of locally compact groups
Comptes Rendus Acad. Sciences, Paris, France vol 267 ser A pp 513-516, 1968

4.   Poisson spaces of Lie groups
Comptes Rendus Acad. Sciences, Paris, France vol 268 serA pp 139-142, 1969

5.   Random walks on locally compact groups
Comptes Rendus Acad. Sciences, Paris, France vol 268 serA.pp 1406-1409, 1969

6.   Poisson Spaces
in Colloque Probability and Algebraic Structures pp 23-31
CNRS Publications Clermont Ferrand France, 1969

7.   (Book) Poisson spaces of locally compact groups  
Lecture Notes Math vol 148, 141 pages, Springer-Verlag, 1970

8.   Anosov’s Diffeomorphisms and Bernouilli schemes
ComptesRendus Acad. Sciences, Paris, France vol 270 ser A pp 1105-1107, 1970

9.   Martin boundaries of random walks on locally compact groups
R. Azencott, P.Cartier 
Proc. 6th Berkeley Symposium on Prob.Stat. vol 3 pp 87-129, 1970

10. Methods of localization and diffusions on manifolds
Publications   Istituto Matematico "Ulisse Dini"  Firenze, Italy, 1971

11. Diffusion processes on differentiable manifold
Comptes Rendus Acad. Sciences,  Paris,  vol 274, pp 651-654, series A, 1972

12. Stability of group representations and Haar spectrum 
R. Azencott, W.Parry 
Trans.Am.Math.Soc. vol 172 pp 317-327, 1972

13. Diffusion processes on differentiable manifold
Comptes Rendus Acad. Sciences, Paris, France, vol 276 serA pp 363-365, 1973

14. Behaviour of diffusion semi-groups at infinity
Bull.Soc.Math.France vol 102   pp 193-240, 1974

15. Homogeneous manifolds with negative curvature
R. Azencott, E.Wilson
Comptes Rendus Acad. Sciences, Paris, France, vol 278 serA pp 561-562, 1974

16. Large Deviations and slow learning processes
R. Azencott, G.Ruget
Comptes Rendus Acad. Sciences, Paris, France vol 281 serA pp 529-532, 1975

17. (Book) Homogeneous manifolds with negative curvature     
R. Azencott, E.Wilson
Memoirs Am.Math.Soc. vol 8  178,   101 pages, 1976

18. Homogeneous manifolds with negative curvature     
R. Azencott, E.Wilson
Trans.Am.Math.Soc. vol 215 pp 323-362, 1976

19. Parameters Estimation through Rank Tests
in "Estimation robuste"   Astérisque, vol 43 pp 41-64,1977

20. Robustness of  R-estimators
in "Estimation robuste"   Astérisque, vol 43 pp 189-202,1977

21. Random mixing of differential equations : Large deviations theory
R. Azencott, G.Ruget
Zeitschrift Wahrscheinlichkeit Geb.  vol 38 pp 1-54, 1977

22. (Book)              Large Deviations theory and Applications          
Invited advanced course at Saint-Flour  Summer school in probability theory    
Lecture Notes Math , vol 774,  176 pages   Springer-Verlag, 1980

23. Diffusions processes on differentiable manifolds
in "Géodésiques et diffusions" Astérisque   vol 84-85   pp 17-32, 1981

24. Local and Global Estimates for  the density of  diffusion processes
in "Géodésiques et diffusions" Astérisque   vol 84-85   pp 131-150, 1981

25. Cramer Transforms and small time diffusions processes
in "Géodésiques et diffusions" Astérisque   vol 84-85   pp 215-226, 1981

26. Diffusion Processes on the Heisenberg group
in "Géodésiques et diffusions" Astérisque  vol 84-85   pp 227-236, 1981

27. (Book)Geodesics and small time behaviour of diffusions on manifolds    
Editor R. Azencott,   series of coordinated papers by 
R.Azencott,  P. Baldi, A. Bellaiche, L. Elie, M. Chaleyat-Maurel)
Asterisque  vol 84-85, 250 pages, Paris, 1982

28. Stochastic Taylor Formula and Feynmann integrals
Lecture Notes Math.vol 921 pp 237-265 Springer-Verlag, 1982

29. (Book)                Séries d’ observations irrégulières       
R. Azencott, D.Dacunha Castelle
1st edition (in french)  190 pages   Masson   Paris, 1983

30. Asymptotic small time expansions for densities of diffusion processes
Lecture Notes Maths vol 1059     pp 402-498    Springer-Verlag, 1984

31. Small random perturbations of dynamic systems :  asymptotic expansions
Bull.Soc.Math.France vol 109     pp 253-308, 1985

32. Schroedinger Equation as « h » tends to zero : a probabilistic approach
R. Azencott, H. Doss in CIRM 1985 colloquium (Editor Albeverio)
Lecture Notes in Maths. Springer-Verlag  vol 1109, 1985

33. Atiyah-Singer Index theorem : Bismut’s approach
Seminaire Bourbaki 1984  Astérisque (Soc. Math. France), 1985

34. (Book) Time Series of irregular observations    
R. Azencott, D. Dacunha Castelle
2nd edition (in english) 198 pages, Springer Verlag, NewYork,1985

35. Simulated Annealing and Image analysis
Publications    Soc.Math.France 16 pages, 1986

36. Gibbs fields ,simulated annealing and low level vision tasks
Congress  « Shape recognition an Artificial intelligence » .  Antibes France
Actes RFIA 87  published by AFCET  Tom II p 1183-1191, 1987

37. (Proceedings)  1987 Internat. Autumn School : Artificial Intelligence 
R. Azencott :   Proceedings Editor  and Scientific Director 
Thomson / Inst. ENS Ulm,  Jouy-en-Josas, France, 1987.

38. Image analysis and Markov fields
Proc Int.Conf on Ind.andAppl.Math. S.I.A.M. 87  Paris
published by, S.I.A.M. Philadelphia, 1988

39. (Proceedings) 1988 Internat.Autumn School: Image Analysis and Artificial Vision 
R. Azencott : Proceedings Editor  and Scientific Director 
Thomson / Inst. ENS Ulm,  Jouy-en-Josas, France, 1988

40. Mandrake, expert software and algorithms for time series forecast
R. Azencott, B. and Y. Girard, R. Astier, P. Jacoubowitz, M.M. Martin
Proceedings of the 8th Int. Symp. on Forecasting, Amsterdam,1988

41. (Book) Time Series of Irregular observations                                       
R. Azencott, D. Dacunha Castelle
3rd edition  (in japanese)   206 pages  Tokyo,1989

42.  (Scientific Software) Mandrake Monovariate :
       Fast monovariate ARIMA models for time series 
R. Azencott, B. et Y. Girard, R. Astier,P.Jacoubowitz,M.M.Martin           
Collaboration between  Univ.Paris 11, Univ. Paris 1, EDF France, 1989

43. Simulated Annealing
Bourbaki Seminar 1988 Lecture 697  Asterisque p 223-237, 1989

44. (Proceedings) 1989 Internat. Autumn School: Image Analysis and Artificial Vision
R. Azencott : Proceedings Editor  and Scientific Director 
Thomson / Inst. ENS Ulm,  Jouy-en-Josas, France, 1989

45. Image compression through backpropagation
R. Azencott, M. Mougeot, B. Angeniol
in "Neuro Computing"      Editors Fogelman-Hierault   
Nato Asi Lecture Notes Series vol F- 68   p 333-336 Springer-Verlag, 1990

46. Synchronous Boltzmann machines and Gibbs fields
in "Neurocomputing" Editors Fogelman-Hierault
NATO ASI Lecture Notes Series vol F68 p 51-62 Springer-Verlag, 1990

47. Synchronous Boltzmann  machines  and artificial vision
"Les Entretiens de Lyon:  Neural Networks" p 135-143    Springer-Verlag, 1990

48. Synchronous Boltzmann machines and outline based image classification      
R. Azencott, A.Doutriaux,L.Younes
Proc Int.Neural Networks Conf Paris1990 IEEE KluwerAc Publ.  Boston, 1990

49. Self-organization of orientation selective cells in the visual cortex
R. Azencott, M.Mougeot    
Congrès Neurosciences   Aussois France,1990

50. (Proceedings) 1990 Internat. Autumn School : Object Oriented Languages
R. Azencott :    Proceedings Editor  and Scientific Director 
Thomson / Inst. ENS Ulm,  Jouy-en-Josas, France, 1990

51. Image compression and back propagation
R. Azencott, B. Angeniol, M. Mougeot 
Neural Networks.  Vol 4,   p 467 - 476, 1991

52. Unsupervised learning for the visual cortex         
R. Azencott, M.Mougeot
Proc Int.J.Conf.Neural Nets    July 1991 Seattle, 1991

53. Contour lines extraction in images by synchronous Boltzmann machines
R. Azencott, J.Lacaille  
Actes Congrès Neuro-Nîmes   p 507 - 518   Nimes France,1991

54. Smooth contour lines in images and synchronous  Boltzmann machines  
R. Azencott, J.Lacaille 
Proceedings Int. J. Cong. Neural Nets, World Scientific  Singapore,1991

55. Detection of seismic horizons : Gibbs fields techniques
R. Azencott, T.Carlier, JL.Guiziou, JF.Yao
6th Conf.Image Analysis and Image Processing  Como Italy 1991    
Published by World Scientific  Singapore, 1992

56. Boltzmann machines : High order interaction and synchronous learning
Proceedings "Stochastic models in image analysis" Rome 1990
Editors P.Barone, A.Frigessi, M. Piccioni
Lecture Notes in Statistics. Vol 74. p 14 - 45 Springer-Verlag, 1992

57. Edge detection and segmentation of textured images
R. Azencott,  C. Graffigne, C. Labourdette 
Proceedings "Stochastic models in image analysis" Rome, 1990
Editors P. Barone, A. Frigessi, M. Piccioni
Lecture Notes in Statistics. Vol 74   Springer-Verlag p 75 - 88, 1992

58. Bayesian 3D path search in Images of seismic data 
R. Azencott, B. Chalmond, P. Julien
Procedings “Stochastic Models in Image Analysis" Rome 1990
Editors P.Barone, A.Frigessi, M. Piccioni
Lecture Notes in Statistics Springer-Verlag Vol 74 p 46 - 74, 1992

59. (Book)      Simulated annealing : Parallelization techniques     
Editor R. Azencott series of coordinated papers by
R. Azencott O. Catoni C. Grafigne L.Younes A. Trouve M. Dreyfus P. Garda H. Lutton  
Interscience 242 pages, Wiley,  New York, 1992

60. Simulated annealing: speed of convergence and acceleration techniques
in "Simulated annealing:parallelization techniques" pp 1-10  Wiley,1992

61. Large deviation framework for sequential and parallel annealing
in "Simulated annealing:parallelization techniques"  pp 11-24   Wiley, 1992

62. Parallel annealing : basic techniques
in "Simulated annealing:parallelization techniques"  pp 37-46   Wiley, 1992

63. Parallel annealing :interacting multiple searches
R. Azencott, C.Graffigne
in "Simulated annealing:parallelization techniques"  pp 81-90   Wiley, 1992

64. Adaptive smoothing and Markov fields for “valleys” detection on Images
R. Azencott, B.Chalmond,F.Coldefy  
11th  IAPR Cong on Image Speech and Signal analysis, La Haye, 1992

65. Non supervised image segmentation and multi-level Markov Random fields
R. Azencott, C. Graffigne
11 th IAPR Cong. Image Speech Signal analysis, La Haye, 1992

66. Synchronous Boltzmann machines and artificial vision
R. Azencott, J. Lacaille, L. Younes                                                                    
Courrier du CNRS, PARIS, vol  79, 1992

67. Markov fields in image analysis : a 1986-1992 survey
University of Paris Sud publications, 1992

68. Non homogeneous transfer function estimation for X-rays films fusion
R.Azencott, J.P. Wang
Preprint 9336, CMLA , ENS Cachan Publications, France, 1993 

69. Synchronous Boltzmann machines and curve identification tasks
R. Azencott, A.Doutriaux, L.Younes
Network (vol  4) pp 461-480,  1993

70. Automated intensive short term forecasts of gas consumptions for France
      national  gas agency GDF (scientific software) 
R. Azencott, B. et Y. Girard,
Collaboration between  ENS Rue d’Ulm,  Univ. Paris 1, GDF France,1993

71. Mandrake Multivariate : parsimonious modeling for multivariate time series
     (scientific software) 
R. Azencott, B. et Y. Girard, J. Lacaille, B. Durand 
Collaboration between  ENS Cachan, Univ. Paris 1, Transports Ministry, 1994

72. Markov fusion of noise blurred  images to detect intensity valleys
R. Azencott, B. Chalmond, F. Coldefy
Int. Jour. Comp. Vision,1994

73. Stochastic Neural Nets applied to optimization  and pattern recognition
Invited advanced course at
Internat. Summer School  EDF / CEA / INRIA, Le Bréau, France,1994

74. Geodesics in diffeomorphisms groups : deformation distance between
      shapes
communication in “Stochastic structures and Monte-Carlo optimisation" 
Internat.Mathematics Congress  Cortona, Italy, 1994

75. Contour lines on 2D Sismic Exploration data : Automated detections of geological
      horizons by Markov random fields (scientific software) 
R. Azencott, T. Carlier, J.F. Yao
validated on Sismic Image data in collaboration with  TOTAL              
conception / realization with Sudimage consultants , Cachan , France, 1995

76. Siredin : Algorithmic simulations  of massively parallel Artificial Retina for
      automatic detection of mobile targets (scientific software) 
R. Azencott, J. Lacaille, P. Garda, O. Cherif, L. Younes, C. Labourdette
scientific collaboration with France Atomic Agency (C.E.A / CESTA)
conception / realization at Miriad Parallel Processing,  Cachan, France, 1995

77. Transfer function estimation, film fusion, and image restauration
R. Azencott, B. Chalmond, J.P. Wang 
CVGIP : Graphical models and image processing 58, pp 65-74, 1996

78. Automated camera detection of leather defects using Markov Random Fields
R. Azencott, J.F. Yao
Proceedings of 12th Int. Congress Pattern Recognition, Jerusalem, 1996

79. A distance for elastic matching in object recognition
R. Azencott,  F. Coldefy, L. Younes
Int. Conf. Pattern Recognition   ICPR 96  Vienna, 1996

80. Multiscale identification of buildings in compressed large aerial scenes 
R. Azencott, F. Durbin, J. Paumard
Int. Conf. Pattern Recognition   ICPR 96  Vienna, 1996

81. Robust recognition in compressed large aerial scenes
R. Azencott, F. Durbin, J. Paumard
Int. Conf. on Image Processing, Lausanne, 1996

82. An energy minimization method for matching structured object representations
R. Azencott, L. Younes
Lecture Notes in Computer Science   vol  1223, 1997

83. Texture classification using windowed Fourier filters
R. Azencott, JP. Wang, L. Younes
IEEE Trans.  Pattern Analy. Machine Intelligence vol 19 n°2   pp 148-152, 1997

84. Large deviations theory and Vapnik exponential bounds for Learning speeds
Invited communication at Brown University 50 th anniversary
Internat. Colloquium , Maths Dept , Brown University, Providence USA, 1997

85. Piecewise affine neural networks and nonlinear control
       R. Azencott, C.A. Lehalle
Proceedings Int. Conf. on Artificial Neural Networks, Skovde, Sweden, 1998

86. Automatic control by piecewise affine neural networks
R. Azencott, C.A. Lehalle
Proceed. “Nonlinear control Theory/Appl.” CransMontana, Switzerland, 1998

87. Miriad Process  :  desktop development tool for automatic learning applied
      to signal analysis (scientific software) 
R. Azencott, J. Lacaille, O. Cherif, O. Catoni, M. Mougeot, B. Durand
validated  on ARIANE rocket engine with France National Space Agency (CNES)
supported by France National Innovation Agency  (ANVAR)
conception/realization at Miriad Parallel Processing,  Cachan, France, 1998

88. Easy Glider : Content driven navigation engine for very large Image/Text
      Databases (scientific software) 
R. Azencott, L.Younes, F. Coldefy, S. Xiong, J. Paumard, JP Wang
validated  on industrial automated image search projects with ALCATEL and CEA
supported by France National Innovation Agency  (ANVAR)
conception/realization at Sudimage SA,   France, 1999

89. Stability of nonlinear control by piecewise affine neural networks
R. Azencott, C.A. Lehalle
Proceedings : Int. Conf. on Artificial Neural Networks   Edinburgh, UK, 1999

90. Piecewise Affine Perceptrons can emulate stable nonlinear controls
R. Azencott, C.A. Lehalle
Proceedings: Int. Symposium  on Intelligent Control  Cambridge  USA, 1999

91. Md Scan : a desktop development tool for automatic learning and pattern
      recognition on multisensors high-rates  signals (scientific software) 
R. Azencott, J. Lacaille, O. Cherif, M. Abitbol, E. Lohmer, B. Durand
validated  for “in flight” acoustic anomaly diagnosis with Airbus and  DASSAULT
conception/realization at Miriad Technologies, Paris, France, 2000

92. Md Monitor: a desktop  development tool for intelligent monitoring and 
      anticipation of  performance indicators (scientific software) 
R. Azencott, J. Lacaille, O. Cherif, M. Abitbol, C. Lehalle, J. Besnard, J.F. Legrand,
validated  on industrial process tele-monitoring  with ALTIS and Schneider     
conception/realization at Miriad Technologies, Paris, France, 2000

93. Refined Exponential Rates in Vapnik-Chervonenkis inequalities
R. Azencott, N. Vayatis
ComptesRendus Acad. Sciences vol 332 serie I, p.563-568   Paris  France, 2001

94. Scan Trends :  Multi-scale  auto-adaptive Trend  Forecast for stockmarket
      data flows (scientific software) 
R. Azencott, J. Lacaille, J. Besnard, C. Lehalle,
validated  on large stockmarket databases in collaboration with CPR  Paris
conception / realization at Miriad Technologies , Paris,  France, 2001

95. Pythie :   On line automatic learning to forecast service levels in cellphone
      networks (scientific software) 
R. Azencott, O. Cherif, J.M. Legrand
validated  on cellphone networks  databases with France TELECOM  and  ALCATEL
conception / realization at Miriad Technologies , Paris,  France, 2001

96. Charts : automated shape recognition for massive detection of celebrated Chartist
      Figures on stockmarket dataflows (scientific software) 
R. Azencott, J. Lacaille, O. Cherif, J. Besnard, C. Lehalle,
validated  on large stockmarket databases in collaboration with DIAGONLINE
conception/realization at Miriad Technologies , Paris,  France, 2001

97. Opacc : auto-adaptive detection of abnormal patterns on massive recordings of
     Hertzian communications (scientific software) 
R. Azencott, J. Besnard, C. Lehalle,
validated  on large Hertzian databases in collaboration with THALES Comm. France
conception/realization at Miriad Technologies , Paris,  France, 2002

98. Scan Welding :  “on line detection of defects in  robotic welding by voltage and
      intensity analysis” (scientific software) 
      R. Azencott,  J. Besnard, C. Lehalle, K. Fakhreddine
validated  on large electric welding databases in collaboration with AIR LIQUIDE
conception/realization at Miriad Technologies , Paris,  France, 2002

99. Simulated Annealing and Image analysis, with comments by J.M. Morel,
R. Azencott  in Selected Papers Soc.Math.France (edit. J.M. Kantor),  2003

100. Md Intelligence (Auto-Adaptive  Decision Tools ) :  Data flow analysis to detect
      anomalies, anticipate criticities, enhance performance (scientific software)      
R. Azencott, J. Lacaille, O. Cherif, M. Abitbol, J. Besnard, J.F. Legrand, C. Lehalle,
validated  on industrial process tele-monitoring with AIR LIQUIDE and  RHODIA,
conception/realization at Miriad Technologies , Paris,  France, 2003

101. On line acoustics identification of sounds in very noisy  background
(scientific software)   R. Azencott, Qu XingTai, J. Besnard , collaboration with   THALES / INRA on telemonitoring systems for  passengers safety in urban buses            Internal Report , THALES / INRA research project, 2004

102. On the fly health monitoring of mechanical hazards in Formula1 car races
R. Azencott, C.A. Lehalle
Proceedings : FISITA World Automotive Congress, 2004

103. Auto adaptive monitoring of complex industrial processes
R. Azencott, C.A. Lehalle
Proceedings : Congress “Fiability of production lines”  (Madrid, Spain) 2004

104. Auto adaptive analysis of engine noise  / vibrations  for online diagnosis
R. Azencott, J. Besnard, C.A. Lehalle
Proceedings :  “Automobile and Railway comfort: Acoustics, Vibratory, Thermal”
International Congress, Douai, France, 2004

105. Multi-sensors detection and diagnosis of defective nodes in
     high throughput multinodes communications networks (scientific software) 
R. Azencott, O. Cherif, J.F. Legrand , collab.Fr Telecom / Ilog, 2005

106. Auto adaptive Kohonen neural maps applied to classification
     and prediction of gas consumers profiles (scientific software)  
R. Azencott, S. Leprince, F. Jodar , collab. GAZ de FRANCE, 2005

107. Support vector machines applied to fast sound identification
      in noisy background  (scientific software)
R. Azencott, Qu XingTai, collab. AIRBUS /SAGEM on tele monitoring 
systems for aircraft passengers cabins, 2006

108. Automated evolution forecast for large patents portfolio (scientific software)
R. Azencott, B. Girard, Y. Girard, 2007  

109. Mutual information  and accidents causation analysis
    R. Azencott, M. Mougeot 
(submitted to “Accident  Analysis and Prevention”), 2007

110. A controllability approach to Shape Identification by elastic matching
R. Azencott, R. Glowinski, A.M. Ramos ;   accepted in
Applied Mathematics Letters, doi.org/10.1016/j.aml.2007.08.013, 2007