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54G20 Counterexamples (69 articles)

• Tkachenko, Mikhail: A countably cellular topological group all of whose countable subsets are closed need not be $\mathbb{R}$-factorizable. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 64 (2023), issue 1, pp. 127-135
• Logunov, Sergei: On butterfly-points in $\beta X$, Tychonoff products and weak Lindelöf numbers. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 63 (2022), issue 3, pp. 379-383
• Logunov, Sergei: On non-normality points, Tychonoff products and Suslin number. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 63 (2022), issue 1, pp. 131-134
• Logunov, Sergei: On Szymański theorem on hereditary normality of $\beta\omega$. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 63 (2022), issue 4, pp. 507-512
• Bielas, Wojciech; Kucharski, Andrzej; Plewik, Szymon: The Niemytzki plane is $\varkappa $-metrizable. (English). Mathematica Bohemica, vol. 146 (2021), issue 4, pp. 457-469
• Logunov, Sergei: Non-normality points and nice spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 62 (2021), issue 3, pp. 383-392
• Logunov, Sergei: On hereditary normality of $\omega ^*$, Kunen points and character $\omega_{1}$. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 62 (2021), issue 4, pp. 507-511
• García-Ferreira, Salvador; Rodríguez-López, Yackelin; Uzcátegui, Carlos: On the continuity of the elements of the Ellis semigroup and other properties. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 62 (2021), issue 2, pp. 225-241
• Tkachuk, Vladimir V.: Exponential domination in function spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 61 (2020), issue 3, pp. 397-408
• Porter, John E.: Monotonically normal $e$-separable spaces may not be perfect. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 59 (2018), issue 3, pp. 391-398
• Castañeda-Alvarado, Enrique; Vidal-Escobar, Ivon: Property of being semi-Kelley for the cartesian products and hyperspaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 58 (2017), issue 3, pp. 359-369
• Rmoutil, Martin: Products of non-$\sigma $-lower porous sets. (English). Czechoslovak Mathematical Journal, vol. 63 (2013), issue 1, pp. 205-217
• Popvassilev, Strashimir G.: Base-base paracompactness and subsets of the Sorgenfrey line. (English). Mathematica Bohemica, vol. 137 (2012), issue 4, pp. 395-401
• Herrlich, Horst; Keremedis, Kyriakos: AC holds iff every compact completely regular topology can be extended to a compact Tychonoff topology. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 52 (2011), issue 1, pp. 139-143
• Hernández-Gutiérrez, Rodrigo; Tamariz-Mascarúa, Angel: Disconnectedness properties of hyperspaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 52 (2011), issue 4, pp. 569-591
• Arhangel'skii, A. V.: A construction of a Fréchet-Urysohn space, and some convergence concepts. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 51 (2010), issue 1, pp. 99-112
• Song, Yan-Kui: Embedding into discretely absolutely star-Lindelöf spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 48 (2007), issue 2, pp. 303-309
• Chodounský, David: Non-normality and relative normality of Niemytzki plane. (English). Acta Universitatis Carolinae. Mathematica et Physica, vol. 48 (2007), issue 2, pp. 37-41
• García-Ferreira, S.; Sanchis, M.: Ultrafilter-limit points in metric dynamical systems. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 48 (2007), issue 3, pp. 465-485
• Tkachenko, M.: Homomorphic images of $\Bbb R$-factorizable groups. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 47 (2006), issue 3, pp. 525-537
• Jordan, Francis; Mynard, Frédéric: Productively Fréchet spaces. (English). Czechoslovak Mathematical Journal, vol. 54 (2004), issue 4, pp. 981-990
• Jelínek, Jiří: A discontinuous function with a connected closed graph. (English). Acta Universitatis Carolinae. Mathematica et Physica, vol. 44 (2003), issue 2, pp. 73-77
• Simon, Petr; Tironi, Gino: No hedgehog in the product?. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 43 (2002), issue 2, pp. 349-361
• Tkachenko, M. G.: Complete $\aleph_0$-bounded groups need not be $\Bbb R$-factorizable. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 42 (2001), issue 3, pp. 551-559
• Arhangel'skii, A. V.; Ludwig, L.: On $\alpha$-normal and $\beta$-normal spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 42 (2001), issue 3, pp. 507-519
• Bonanzinga, M.; Matveev, M. V.: Centered-Lindelöfness versus star-Lindelöfness. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 41 (2000), issue 1, pp. 111-122
• Arhangel'skii, A.: Moscow spaces, Pestov-Tkačenko Problem, and $C$-embeddings. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 41 (2000), issue 3, pp. 585-595
• Charatonik, Marta N.; Charatonik, Włodzimierz J.: On Mazurkiewicz sets. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 41 (2000), issue 4, pp. 817-819
• van Hartskamp, Michael; van Mill, Jan: Some examples related to colorings. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 41 (2000), issue 4, pp. 821-827
• Pyrih, Pavel: An example for mappings related to confluence. (English). Archivum Mathematicum, vol. 35 (1999), issue 2, pp. 145-149
• Costantini, Camillo: On a problem of Nogura about the product of Fréchet-Urysohn $\langle \alpha_4\rangle$-spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 40 (1999), issue 3, pp. 537-549
• Simon, Petr: A hedgehog in a product. (English). Acta Universitatis Carolinae. Mathematica et Physica, vol. 39 (1998), issue 1, pp. 147-153
• Bell, Murray: A compact ccc non-separable space from a Hausdorff gap and Martin's Axiom. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 37 (1996), issue 3, pp. 589-594
• Vaughan, Jerry E.: A countably compact, separable space which is not absolutely countably compact. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 36 (1995), issue 1, pp. 197-201
• Daniels, Peg: $\omega^\omega$-directedness and a question of E. Michael. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 36 (1995), issue 1, pp. 115-121
• Thümmel, Egbert: The fixed point set of open mappings on extremally disconnected spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 35 (1994), issue 4, pp. 789-794
• Ismail, Mohammad: A topological characterization of stationary sets. (English). Czechoslovak Mathematical Journal, vol. 43 (1993), issue 4, pp. 765-767
• Schröder, J.: Natural sinks on $Y_\beta$. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 33 (1992), issue 1, pp. 173-179
• Bentley, H. L.; Lowen-Colebunders, E.: Completely regular spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 32 (1991), issue 1, pp. 129-153
• Bell, Murray G.: Not all dyadic spaces are supercompact. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 31 (1990), issue 4, pp. 775-779
• Leiderman, A. G.: Adequate families of sets and function spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 29 (1988), issue 1, pp. 31-39
• Simon, Petr; Tironi, Gino: Two examples of pseudo-radial spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 27 (1986), issue 1, pp. 155-161
• Hart, Klaas Pieter; Junnila, Heikki; van Mill, Jan: A Dowker group. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 26 (1985), issue 4, pp. 799-810
• Simon, Petr; Weese, Martin: Nonisomorphic thin-tall superatomic Boolean algebras. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 26 (1985), issue 2, pp. 241-252
• Simon, Petr: Completely regular modification and products. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 25 (1984), issue 1, pp. 121-128
• Williams, Scott W.: Paracompactness in box products. (English). In: Frolík, Z. (ed.): Proceedings of the 12th Winter School on Abstract Analysis. Section of Topology. Circolo Matematico di Palermo, Palermo, 1984. pp. [313]-328
• Hušek, Miroslav; Pelant, Jan: Uniform weight of uniform quotients. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 24 (1983), issue 2, pp. 335-340
• Juhász, István; van Mill, Jan: Countably compact spaces all countable subsets of which are scattered. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 22 (1981), issue 4, pp. 851-855
• Trnková, Věra: Homeomorphisms of powers of metric spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 21 (1980), issue 1, pp. 41-53
• van Douwen, Eric K.: Nonsupercompactness and the reduced measure algebra. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 21 (1980), issue 3, pp. 507-512
• Neubrunn, Tibor: Erratum: ``A note on mappings of Baire spaces' (Math. Slovaca 27 (1977), no. 2, 173--176). (English). Mathematica Slovaca, vol. 27 (1977), issue 4, pp. 442
• Wage, Michael L.: Spaces with zero set bases. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 18 (1977), issue 4, pp. 771-775
• Malykhin, Viacheslav I.: Oб одной задаче Э. Чеха. (Russian) [On a problem of E. Čech]. Commentationes Mathematicae Universitatis Carolinae, vol. 17 (1976), issue 4, pp. 779-790
• Rajagopalan, M.: Compact C-spaces and S-spaces. (English). In: General topology and its relations to modern analysis and algebra IV, Proceedings of the fourth Prague topological symposium, 1976. Part A: Invited papers. Springer, Berlin, 1977. pp. [179]-189
• Mioduszewski, J.: Compact Hausdorff spaces with two open sets. (English). In: Novák, Josef (ed.): General topology and its relations to modern analysis and algebra IV, Proceedings of the fourth Prague topological symposium, 1976. Part B: Contributed Papers. Society of Czechoslovak Mathematicians and Physicist, Praha, 1977. pp. [288]-290
• Lukeš, Jaroslav; Zajíček, Luděk: Fine topologies as examples of non-Blumberg Baire spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 17 (1976), issue 4, pp. 683-688
• Pol, Elżbieta; Pol, Roman: A hereditarily normal strongly zero-dimensional space containing subspaces of arbitrary large dimension. (English). In: Novák, Josef (ed.): General topology and its relations to modern analysis and algebra IV, Proceedings of the fourth Prague topological symposium, 1976. Part B: Contributed Papers. Society of Czechoslovak Mathematicians and Physicist, Praha, 1977. pp. [357]-360
• Wage, M. L.: On a problem of Katětov. (English). In: Novák, Josef (ed.): General topology and its relations to modern analysis and algebra IV, Proceedings of the fourth Prague topological symposium, 1976. Part B: Contributed Papers. Society of Czechoslovak Mathematicians and Physicist, Praha, 1977. pp. [494]-499
• van Douwen, Eric K.: A technique for constructing examples. (English). In: Novák, Josef (ed.): General topology and its relations to modern analysis and algebra IV, Proceedings of the fourth Prague topological symposium, 1976. Part B: Contributed Papers. Society of Czechoslovak Mathematicians and Physicist, Praha, 1977. pp. 111-112
• Pelant, Jan: General hedgehogs in general topology. (English). In: Frolík, Zdeněk (ed.): Seminar Uniform Spaces. , 1976. pp. 145-150
• Pelant, Jan; Pták, Pavel: On $\sigma$-discreteness in uniform spaces. (English). In: Frolík, Zdeněk (ed.): Seminar Uniform Spaces. , 1976. pp. 115-119
• Rödl, Vojtěch: Fineness in the category of all 0-dimensional uniform spaces. (English). In: Frolík, Zdeněk (ed.): Seminar Uniform Spaces. , 1975. pp. 139-143
• Frič, Roman: Further note on Fréchet spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 14 (1973), issue 4, pp. 661-667
• Pelant, Jan: Reflections not preserving completeness. (English). In: Frolík, Zdeněk (ed.): Seminar Uniform Spaces. , 1975. pp. 235-240
• Frič, Roman: A note on Fréchet spaces. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 13 (1972), issue 3, pp. 411-418
• Simon, Petr: A note on Rudin's example of Dowker space. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 12 (1971), issue 4, pp. 825-834
• Simon, P.: A note on Rudin's sxample of Dowker space. (English). In: Novák, J. (ed.): General Topology and its Relations to Modern Analysis and Algebra. Proceedings of the Third Prague Topological Symposium, 1971. Academia Publishing House of the Czechoslovak Academy of Sciences, Praha, 1972. pp. 399-400
• Kannan, V.; Rajagopalan, M.: On rigidity and groups of homeomorphisms. (English). In: Novák, J. (ed.): General Topology and its Relations to Modern Analysis and Algebra. Proceedings of the Third Prague Topological Symposium, 1971. Academia Publishing House of the Czechoslovak Academy of Sciences, Praha, 1972. pp. 231-234
• Franklin, S. P.: A homogeneous Hausdorff $E_0$-space which is not $E_1$. (English). In: Franklin, S. P., Frolík, Z. and Koutník, V. (eds.): General Topology and Its Relations to Modern Analysis and Algebra. Proceedings of the Kanpur topological conference, 1968. Academia Publishing House of the Czechoslovak Academy of Sciences, Praha, 1971. pp. 125-126