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Localic real functions: A general setting. (English) Zbl 1187.06005

Given a topological space \(X\), it is sometimes desirable to consider real-valued functions on \(X\) which are not necessarily continuous. For instance, a topological space \(X\) is said to be a Blumberg space if for every (not necessarily continuous) function \(f\colon X \to \mathbb{R}\) there is a dense subspace \(D\) of \(X\) such that the restriction \(f_{|D} \colon D\to\mathbb{R}\) is continuous. In pointfree topology, the only real-valued functions on a frame \(L\) that have hitherto been considered are the continuous and the semicontinuous ones. In this paper, the authors (continuing their programme of extending to the pointfree context real-valued functions [see Algebra Univers. 60, No. 2, 169–184 (2009; Zbl 1181.06003); J. Pure Appl. Algebra 212, No. 5, 955–968 (2008; Zbl 1133.06008); J. Pure Appl. Algebra 213, No. 1, 98–108 (2009; Zbl 1154.06006)]) define a real-valued function on a frame as follows. Starting with a frame \(L\), they take an isomorphic copy of the assembly of \(L\) in the form of the lattice of sublocales of \(L\) turned upside down and denote it by \(\mathcal{S}(L)\). Then, with \(\mathfrak{L}(\mathbb{R})\) denoting the frame of reals, they regard the lattice-ordered ring \(\mathbf{Frm}(\mathfrak{L}(\mathbb{R}), \mathcal{S}(L))\) as the frame analogue of the ring of all real-valued functions on a topological space. The continuous (respectively, lower and upper semicontinuous) functions in \(\mathbf{Frm}(\mathfrak{L}(\mathbb{R}), \mathcal{S}(L))\) are defined akin to topology; to wit, a function \(F\) in \(\mathbf{Frm}(\mathfrak{L}(\mathbb{R}), \mathcal{S}(L))\) is continuous if \(F(p, q)\) is a closed sublocale for all \((p, q) \in\mathfrak{L}(\mathbb{R})\). Semicontinuous functions are defined similarly. Appended to the end of the paper are much more lucid formulations of the authors’ results from their papers cited above, expressed in terms of ideas developed in the current paper.

MSC:

06D22 Frames, locales
26A15 Continuity and related questions (modulus of continuity, semicontinuity, discontinuities, etc.) for real functions in one variable
54D15 Higher separation axioms (completely regular, normal, perfectly or collectionwise normal, etc.)

References:

[1] Aumann, G., Reelle Funktionen (1974), Springer: Springer Berlin · Zbl 0056.05202
[2] Banaschewski, B., (The Real Numbers in Pointfree Topology. The Real Numbers in Pointfree Topology, Textos de Matemática, Série B, vol. 12 (1997), Universidade de Coimbra) · Zbl 0891.54009
[3] Banaschewski, B.; Mulvey, C. J., Stone-Čech compactification of locales II, J. Pure Appl. Algebra, 33, 107-122 (1984) · Zbl 0549.54017
[4] Dilworth, R. P., The normal completion of the lattice of continuous functions, Trans. Amer. Math. Soc., 68, 427-438 (1950) · Zbl 0037.20205
[5] Dowker, C. H.; Papert, D., On Urysohn’s Lemma, (Proc. of the Second Topological Symposium, Prague, 1966 (1967), Academia: Academia Prague), 111-114 · Zbl 0162.54601
[6] Dowker, C. H.; Papert Strauss, D., Paracompact frames and closed maps, Symposia Math., 16, 93-116 (1975) · Zbl 0324.54015
[7] Gillman, L.; Jerison, M., Rings of Continuous Functions (1976), Springer: Springer New York · Zbl 0151.30003
[8] Gutiérrez García, J.; Kubiak, T.; Picado, J., Monotone insertion and monotone extension of frame homomorphisms, J. Pure Appl. Algebra, 212, 955-968 (2008) · Zbl 1133.06008
[9] J. Gutiérrez García, T. Kubiak, J. Picado, Lower and upper regularizations of frame semicontinuous real functions, Algebra Universalis (2009), doi:10.1017/s00012-009-2102-8; J. Gutiérrez García, T. Kubiak, J. Picado, Lower and upper regularizations of frame semicontinuous real functions, Algebra Universalis (2009), doi:10.1017/s00012-009-2102-8
[10] Gutiérrez García, J.; Kubiak, T.; Picado, J., Pointfree forms of Dowker’s and Michael’s insertion theorems, J. Pure Appl. Algebra, 213, 98-108 (2009) · Zbl 1154.06006
[11] Gutiérrez García, J.; Kubiak, T.; de Prada Vicente, M. A., Insertion of lattice-valued and hedgehog-valued functions, Topology Appl., 153, 1458-1475 (2006) · Zbl 1094.54009
[12] Gutiérrez García, J.; Kubiak, T.; de Prada Vicente, M. A., Generating and inserting continuous functions with values in bounded complete domains and hedgehog-like structures, Houston J. Math., 34, 123-143 (2008) · Zbl 1160.54012
[13] Gutiérrez García, J.; Picado, J., On the algebraic representation of semicontinuity, J. Pure Appl. Algebra, 210, 299-306 (2007) · Zbl 1117.06006
[14] Johnstone, P. T., Stone Spaces (1982), Cambridge Univ. Press: Cambridge Univ. Press Cambridge · Zbl 0499.54001
[15] Kubiak, T., A strengthening of the Katětov-Tong insertion theorem, Comment. Math. Univ. Carolinae, 34, 357-362 (1993) · Zbl 0807.54023
[16] Li, Y.-M.; Wang, G.-J., Localic Katětov-Tong insertion theorem and localic Tietze extension theorem, Comment. Math. Univ. Carolinae, 38, 801-814 (1997) · Zbl 0938.06008
[17] Picado, J., A new look at localic interpolation theorems, Topology Appl., 153, 3203-3218 (2006) · Zbl 1104.06007
[18] Picado, J.; Pultr, A., Sublocale sets and sublocale lattices, Arch. Math. (Brno), 42, 409-418 (2006) · Zbl 1164.06313
[19] Picado, J.; Pultr, A.; Tozzi, A., Locales, (Pedicchio, M. C.; Tholen, W., Categorical Foundation — Special Topics in Order, Algebra and Sheaf Theory. Categorical Foundation — Special Topics in Order, Algebra and Sheaf Theory, Encyclopedia of Mathematics and its Applications, vol. 97 (2003), Cambridge Univ. Press: Cambridge Univ. Press Cambridge), 49-101 · Zbl 1080.06010
[20] Schmid, J., Rational extension of \(C(X)\) and semicontinuous functions, Dissert. Math., 270, 1-27 (1980) · Zbl 0675.54017
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