Seppo Hassi
Department of Mathematics and Statistics, University of Vaasa

Some recent publications
  • S. Hassi, H.S.V. de Snoo, and H. Winkler, Boundary-value problems for two-dimensional canonical systems, Integral Equations Operator Theory, 36 (2000), 445--479.
  • S. Hassi, C. Remling, and H.S.V. de Snoo, Subordinate solutions and spectral measures of canonical systems, Integral Equations Operator Theory, 37 (2000), 48--63.
  • A. Fleige, S. Hassi, and H.S.V. de Snoo, A Kre\u\i n space approach to representation theorems and generalized Friedrichs extensions, Acta Sci. Math. (Szeged), 66 (2000), 595--612.
  • V.A. Derkach, S. Hassi, M.M. Malamud, and H.S.V. de Snoo, Generalized resolvents of symmetric operators and admissibility, Methods of Functional Analysis and Topology, 6, No. 3 (2000), 24--55.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, Generalized Nevanlinna functions with polynomial asymptotic behaviour at infinity and regular perturbations, (Operator Theory and Analysis. The M.A. Kaashoek Anniversary Volume.), Oper. Theory Adv. Appl., 122 (2001), 169--189.
  • S. Hassi, H.S.V. de Snoo, and E.R.Tsekanovski\u\i, An addendum to the multiplication and factorization theorems of Brodski\u\i-Liv\v{s}ic-Potapov, Applicable Analysis, 77 (2001), 125--133.
  • S. Hassi, H.S.V. de Snoo, and E.R. Tsekanovski\u\i, On commutative and noncommutative representations of matrix-valued Herglotz-Nevanlinna functions, Applicable Analysis, 77 (2001), 135--147.
  • V.A. Derkach, S. Hassi, M.M. Malamud, and H.S.V. de Snoo, Weyl functions and intermediate extensions of symmetric operators, Reports Nat. Acad. Sci. Ukraine (Dopov. Nats. Akad. Nauk. Ukraini), 10 (2001), 33--39.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, Operator models associated with singular perturbations, Methods of Functional Analysis and Topology, 7, No. 3 (2001), 1--21.
  • Yu. Arlinski\u\i, S. Hassi, Z. Sebesty\'en, and H.S.V. de Snoo, On the class of extremal extensions of a nonnegative operator, (Recent Advances in Operator Theory and Related Topics. The B\'ela Sz\"okefalvi-Nagy Memorial Volume.), Oper. Theory Adv. Appl., 127 (2001), 41--81.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, Rank one perturbations in a Pontryagin space with one negative square, J. Funct. Anal., 188 (2002), 317--349.
  • S. Hassi, H.S.V. de Snoo, and E.R. Tsekanovski\u\i, Realizations of Herglotz-Nevanlinna functions via $F$-systems, Oper. Theory Adv. Appl., 132 (2002), 183--198.
  • S. Hassi and L.L. Oridoroga, Theorem of completeness for a Dirac type operator with generalized $\lambda$-depending boundary conditions, Spectral and Evolutionary Problems, 12 (2002), 161--166.
  • S. Belyi, S. Hassi, H.S.V. de Snoo, and E.R. Tsekanovski\u\i, On the realization of inverse Stieltjes functions, Electronic Proceedings of 15th International Symposium on the Mathematical Theory of Networks and Systems (Editors: D.S. Gilliam and J. Rosenthal), 2002 (11 pages).
  • S. Hassi, M. Möller, and H.S.V. de Snoo, Differential operators and spectral functions, Proceedings of Ukrainian Congress of Mathematics - Kiev 2001, Proceedings of UMC-2001, 11 (2002), 313--322.
  • Yu. Arlinski\u\i, S. Hassi, H.S.V. de Snoo, and E.R. Tsekanovski\u\i, One-dimensional perturbations of selfadjoint operators with finite or discrete spectrum, Contemporary Mathematics, 323 (2003), 419--433.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, Singular perturbations of selfadjoint operators, Mathematical Physics, Analysis and Geometry, 6 (2003) 349--384.
  • S. Hassi and L. L. Oridoroga, Completeness theorems for Dirac-type operators with boundary conditions of general form depending on the spectral parameter, Math. Notes, 74 (2003), 302--307.
  • V.A. Derkach and S. Hassi, A reproducing kernel space model for $\mathbf{N}_\kappa$-functions, Proc. Amer. Math. Soc., 131 (2003), 3795--3806.
  • S. Hassi, M. Möller, and H.S.V. de Snoo, Sturm-Liouville operators and their spectral functions, J. Math. Anal. Appl., 282 (2003), 584--602.
  • S. Hassi, On the Friedrichs and the Kre\u{\i}n-von Neumann extension of nonnegative relations, In Contributions to Management Science, Mathematics and Modelling. Essays in Honour of Professor Ilkka Virtanen. Eds. Matti Laaksonen and Seppo Pynnönen. Acta Wasaensia 122 (2004), 37--54.
  • S. Hassi, H.S.V. de Snoo, and E.R.Tsekanovski\u{\i}, The realization problem for Herglotz-Nevanlinna functions, In Unsolved Problems in Mathematical Systems and Control Theory (as Problem 1.2), Princeton University Press (2004), 8--13.
  • S. Hassi, M.M. Malamud, and H.S.V. de Snoo, On Krein's extension theory of nonnegative operators, Math. Nachr., 274/275 (2004), 40--73.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, Singular perturbations as range perturbations in a Pontryagin space, Oper. Theory Adv. Appl., 154 (2004), 51--68.
  • S. Hassi and I. Karabash, Similarity of J-selfadjoint Sturm-Liouville operators with an operator potential to selfadjoint ones, Spectral and evolutionary problems, (Proceedings of the Fourteenth Crimenian Autumn Mathematical School-Symposium), 14 (2004), 44--46.
  • S. Hassi, M. Möller, and H.S.V. de Snoo, Singular Sturm-Liouville problems whose coefficients depend rationally on the eigenvalue parameter, J. Math. Anal. Appl., 295 (2004), 258--275.
  • V.A. Derkach, S. Hassi, M.M. Malamud, and H.S.V. de Snoo, Boundary relations and the Weyl families, Rus. Doklady Acad. Sci., 399, No. 2 (2004), 151--156.
  • Yu. Arlinski\u{\i}, S. Hassi, and H.S.V. de Snoo, Q-functions of Hermitian contractions of Krein-Ovcharenko type, Integral Equations and Operator Theory, 53 (2005), 153--189.
  • S. Hassi and I. Karabash, Similarity between J-self-adjoint Sturm-Liouville operators with an operator potential and self-adjoint operators, Math. Notes, 78 (2005), 581--585.
  • S. Hassi, Z. Sebesty\'en, and H.S.V. de Snoo, On the nonnegativity of operator products, Acta Math. Hungar., 109 (1-2) (2005), 1--14.
  • S. Hassi, M.M. Malamud, and  V.I. Mogilevskii, Generalized resolvents and boundary triplets for dual pairs of linear relations, Methods of Functional Analysis and Topology, 11, No 2 (2005), 170--187.
  • S. Hassi, A. Sandovici, H.S.V. de Snoo, and H. Winkler, Extremal extensions of nonnegative linear relations, In: Proceedings of the AIT 2005 conference, Vaasa. Proceedings of the University of Vaasa, Reports 124 (2005), 57--66.
  • V.A. Derkach, S. Hassi, M.M. Malamud, and H.S.V. de Snoo, Boundary relations and orthogonal coupling of symmetric operators, In: Proceedings of the AIT 2005 conference, Vaasa. Proceedings of the University of Vaasa, Reports 124 (2005), 41--56.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, A factorization model for the generalized Friedrichs extension in a Pontryagin space, Oper. Theory Adv. Appl., 162 (2005), 117--133.
  • A. Fleige, S. Hassi, H.S.V. de Snoo, and H. Winkler, Generalized Friedrichs extensions associated with interface conditions for Sturm-Liouville operators, Oper. Theory Adv. Appl., 163 (2005), 135--145.
  • Yu. Arlinski\u{\i}, S. Hassi, and H.S.V. de Snoo, Q-functions of quasi-selfadjoint contractions, Oper. Theory Adv. Appl., 163 (2005), 23--54.
  • S. Hassi, A. Sandovici, H.S.V. de Snoo, and H. Winkler, Form sums of nonnegative selfadjoint operators, Acta Math. Hungar., 111 (1-2) (2006), 81--105.
  • S. Hassi and L.L. Oridoroga, Completeness and Riesz basis property of systems of eigenfunctions and associated functions of Dirac-type operators with boundary conditions depending on the spectral parameter, Math. Notes 79, No. 4 (2006), 589--593.
  • S. Hassi and A. Luger, Generalized zeros and poles of $\mathcal{N}_\kappa$-functions: On the underlying spectral structure, Methods of Functional Analysis and Topology, 12, No 2 (2006), 131--150.
  • S. Belyi, S. Hassi, H.S.V. de Snoo, and E.R. Tsekanovski\u{\i}, A general realization theorem for matrix-valued Herglotz-Nevanlinna functions, Linear Algebra Appl., 419 (2006), 331--358.
  • S. Hassi, M. Möller, and H.S.V. de Snoo, A subclass of Nevanlinna functions associated with Sturm-Liouville operators, Proc. Amer. Math. Soc., 134, No. 10 (2006), 2885--2893.
  • V.A. Derkach, S. Hassi, M.M. Malamud, and H.S.V. de Snoo, Boundary relations and their Weyl families, Trans. Amer. Math. Soc., 358, No. 12 (2006), 5351--5400.
  • S. Hassi, H.S.V. de Snoo, A. Sterk, and H. Winkler, Non-standard boundary conditions for a class of Sturm-Liouville problems, Revue Roumaine de Mathematiques Pures et Appliquees (Romanian Journal of Pure and Applied Mathematics), 51, No. 5--6 (2006), 641--653.
  • S. Hassi, A. Sandovici, H.S.V. de Snoo, and H. Winkler, Extremal extensions for the sum of nonnegative selfadjoint relations, Proc. Amer. Math. Soc., 135 (2007), 3193--3204.
  • V.A. Derkach, S. Hassi, and H.S.V. de Snoo, Asymptotic expansions of generalized Nevanlinna functions and their spectral properties, Oper. Theory Adv. Appl., 175 (2007), 51--88.
  • S. Hassi, H.S.V. de Snoo, and H. Winkler, On exceptional extensions close to the generalized Friedrichs extension of symmetric operators, Oper. Theory Adv. Appl., 175 (2007), 111--120.
  • S. Hassi, H.S.V. de Snoo, A. Sterk, and H. Winkler, Finite-dimensional graph perturbations of selfadjoint Sturm-Liouville operators, in: Operator Theory, Structured Matrices, and Dilations (Tiberiu Constantinescu Memorial Volume), Theta Series in Advanced Mathematics, (2007), 205--228.
  • S. Hassi, Z. Sebesty\'en, H.S.V. de Snoo, and F.H. Szafraniec, A canonical decomposition for linear operators and linear relations, Acta Math. Hungar., 115, No. 4 (2007), 281--307.
  • S. Hassi, A. Sandovici, H.S.V. de Snoo, and H. Winkler, A general factorization approach to the extension theory of nonnegative operators and relations, J. Operator Theory, 58, No. 2 (2007), 351--386.
  • Yu. Arlinski\u{\i}, S. Hassi, and H.S.V. de Snoo, Parametrization of contractive block-operator matrices and passive discrete-time systems, Complex Analysis and Operator Theory, 1 (2007), 211--233.
  • J. Behrndt, S. Hassi, and H.S.V. de Snoo, Functional models for Nevanlinna families, Opuscula Mathematica, 28, No. 3 (2008), 233--245.
  • S. Hassi, A. Sandovici, H.S.V. de Snoo, and H. Winkler, One-dimensional perturbations, asymptotic expansions, and spectral gaps, Oper. Theory Adv. Appl., 188 (2008), 149--173.
  • S. Hassi and S. Kuzhel, On symmetries in the theory of finite rank singular perturbations, J. Functional Analysis, 256 (2009), 777--809. (Published online 28 November 2008.)
  • Yu. Arlinski\u{\i}, S. Hassi, and H.S.V. de Snoo, Passive systems with a normal main operator and quasi-selfadjoint systems, Complex Analysis and Operator Theory, 3 (2009), 19--56.
  • J. Behrndt, S. Hassi, and H.S.V. de Snoo, Boundary relations, unitary colligations, and functional models, Complex Analysis and Operator Theory, 3 (2009), 57--98.
  • V.A. Derkach, S. Hassi, M.M. Malamud, and H.S.V. de Snoo, Boundary relations and generalized resolvents of symmetric operators, Russ. J. Math. Phys., 16, No. 1 (2009), 17--60.
  • S. Hassi and L.L. Oridoroga, Theorem of completeness for a Dirac-type operator with generalized $\lambda$-depending boundary conditions, Integral Equations and Operator Theory, 64 (2009), 357--379.
  • S. Hassi, Z. Sebesty\'en, and H.S.V. de Snoo, Domain and range descriptions for adjoint relations, and parallel sums and differences of forms, Oper. Theory Adv. Appl. (to appear).
  • A. Fleige, S. Hassi, H.S.V. de Snoo, and  H. Winkler, Sesquilinear forms corresponding to a non-semibounded Sturm-Liouville operator, Proc. Roy. Soc. Edinburgh Sect. A (to appear).
  • S. Hassi, H.S.V. de Snoo, and F.H. Szafraniec, Componentwise and Cartesian decompositions of linear relations, Dissertationes Mathematicae, (54 p.) (to appear).
  • S. Hassi, Z. Sebesty\'en, and H.S.V. de Snoo, Lebesgue type decompositions for nonnegative forms, J. Functional Analysis (to appear).

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