Abstract
This paper is a contribution toward developing a theory of expansions of semi-Heyting algebras. It grew out of an attempt to settle a conjecture we had made in 1987. Firstly, we unify and extend strikingly similar results of [48] and [50] to the (new) equational class DHMSH of dually hemimorphic semi-Heyting algebras, or to its subvariety BDQDSH of blended dual quasi-De Morgan semi-Heyting algebras, thus settling the conjecture. Secondly, we give a criterion for a unary expansion of semi-Heyting algebras to be a discriminator variety and give an algorithm to produce discriminator varieties. We then apply the criterion to exhibit an increasing sequence of discriminator subvarieties of BDQDSH. We also use it to prove that the variety DQSSH of dually quasi-Stone semi- Heyting algebras is a discriminator variety. Thirdly, we investigate a binary expansion of semi-Heyting algebras, namely the variety DblSH of double semi-Heyting algebras by characterizing its simples, and use the characterization to present an increasing sequence of discriminator subvarieties of DblSH. Finally, we apply these results to give bases for “small” subvarieties of BDQDSH, DQSSH, and DblSH.
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Dedicated to My mother Yankawwa and My father Pandappa
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Sankappanavar, H.P. Expansions of Semi-Heyting Algebras I: Discriminator Varieties. Stud Logica 98, 27–81 (2011). https://doi.org/10.1007/s11225-011-9322-6
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DOI: https://doi.org/10.1007/s11225-011-9322-6
Keywords
- Dually hemimorphic semi-Heyting algebra
- Dually pseudocomplemented semi-Heyting algebra
- De Morgan semi-Heyting algebra
- Blended ∨-De Morgan law
- Blended dually quasi-De Morgan semi-Heyting algebra
- Blended dually quasi-Stone semi-Heyting algebra
- Double semi-Heyting algebra
- congruence
- normal filter
- discriminator variety
- simple
- directly indecomposable
- subdirectly irreducible
- equational base