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 Books and Monographs Unbounded Operator Algebras and Representation Theory. Akademie-Verlag Berlin and Birkhäuser-Verlag Basel, 1990. 380pp (with A.U. Klimyk) Quantum Groups and Their Representations. Texts and Monographs in Physics, Springer-Verlag Heidelberg, 1997. 552 pp. Errata Unbounded self-adjoint operators on Hilbert space. Graduate Texts in Mathematics, 265. Springer, Dordrecht, 2012. xx+432 pp. Publications     Noncommutative real algebraic geometry A strict Positivstellensatz for the Weyl algebra. Math. Ann. 331, 779-794; arxiv A strict Positivstellensatz for Enveloping algebras. Math. Z. 254 (2006) 641-653; Erratum: Math. Z. 259(2008) 231; arxiv Noncommutative real algebraic geometry—some basic concepts and first ideas. Emerging applications of algebraic geometry, 325–350, IMA Vol. Math. Appl., 149, Springer, New York, 2009; arxiv Algebras of fractions and strict Positivstellensätze for ∗-algebras. J. Reine Angew. Math. 647 (2010), 57–86.; arxiv (with Yu. Savchuk) A noncommutative version of the Fejér-Riesz theorem.  Proc. Amer. Math. Soc. 138 (2010), 1243–1248; arxiv (with Yu. Savchuk) Positivstellensätze for algebras of matrices. Linear Algebra Appl. 436 (2012), 758–788. Around Hilbert's 17th problem. In Optimization stories, Editor M. Grötschel, Documenta Mathematica, Extra Volume ISMP (2012) 433–438. (with J. Cimpric and Yu. Savchuk), On q-normal operators and quantum complex plane. To appear in Trans. Amer. Math. Soc. [arXiv] Quantum groups and noncommutative geometry (with A. Schüler) Classification of bicovariant differential calculi on quantum groups of type A,B,C and D. Commun. Math. Phys. 167, 635-670 (1995). (with A.Schüler) Classification of bicovariant differential calculi on quantum linear groups. Commun. Math. Phys. 170, 315-335 (1995). (with A. Schüler) Left-covariant differential calculi on SLq(2) and SLq(3). J. Geom. Phys. 20, 87-115 (1996) (with I. Heckenberger) Levi-Civita connections on the quantum groups SLq(N), Oq(N) and Spq(N). Commun. Math. Phys. 185, 177-196 (1997) (with I. Heckenberger) Classification of bicovariant differential calculi on the quantum groups SLq(n+1) and Spq(2n). J. Reine Angew. Math. 502, 141-162 (1998) Commutator representations of differential calculi on the quantum group SUq(2). J. Geom. Phys. 31, 241-264 (1999) A polar decomposition for holomorphic functions on a strip. Bull. London Math. Soc.  33, 309-319 (2001) Commutator representations of covariant differential calculi on quantum groups. Lett. Math. Phys. 59, 95-106 (2002) The quantum quarter plane and the real quantum plane. Internat. J. Math. 13, 279-321 (2002) (with E. Wagner) Examples of twisted cyclic cocycles from covariant differetial calculi. Lett. Math. Phys. 64, 245-254 (2003) (with E. Wagner) Dirac operator and a twisted cyclic cocycle on the standard Podles quantum sphere. J. Reine Angew. Math. 574, 219-235 (2004) (with V. L. Ostrovskyi) A resolvent approach to the real quantum plane. Preprint, 2013. Unbounded operators and unbounded representations Integrable operator representations of R_q2, Xq, and SLq(2,R). Commun. Math. Phys. 159, 217-237 (1994) Operator representations of Uq(sl_(R)). Lett. Math. Phys. 37, 211-222 (1996) Operator representations of a q-deformed Heisenberg algebra. J. Math. Phys. 40, 4596-4604 (1999) On well-behaved representations of *-algebras. J. Operator Theory 48, 287-502 (2002) (with S. Ota) Some self-adjoint 2x2 operator matrices associated with closed operators. Integral Equations Operator Theory 45, 475-484 (2003) (with E. Wagner) Hilbert space representations of cross product algebras. J. Funct. Anal.  200, 451-493 (2003) (with E. Wagner) Hilbert space representations of cross product algebras II. Algebr. Repres. Theory 9(2006) 431-464; arXiv math.GA/0308229 (with E. Wagner) Representations of cross product algebras with Podles spheres, J. Lie Theory 17(2007)751-790; arXiv math.QA/0305309 (with Yu. Savchuk) Unbounded Induced Representations of *-Algebras. To appear in Algebras and Representation Theory; arxiv  (with M. M. Malamud) Spectral theory of Schrödinger operators with infinitely many point interactions and radial positive definite functions. J. Funct. Anal. 263 (2012), 3144–3194. arxiv Multidimensional moment problems The K-moment problem for compact semi-algebraic sets. Math. Ann. 289, 203-206 (1991) On determinacy notions for the two dimensional moment problem. Arkiv für Mat. 29, 277-284 (1991) Non-commutative moment problems. Math. Z. 206, 623-650 (1991) On the moment problem of closed semi-algebraic sets. J. Reine Angew. Math. 558, 225-234 (2003) (with M. Putinar) Multivariate Determinateness. Indiana Univ. Math. J. 57, 2931-2986 (2008), arxiv