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Dirichlet is not just bad and singular

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JournalAdvances in Mathematics
DateAccepted/In press - 18 Feb 2022
DateE-pub ahead of print (current) - 17 Mar 2022
Number of pages57
Early online date17/03/22
Original languageEnglish

Abstract

It is well known that in dimension one the set of Dirichlet improvable real numbers consists precisely of badly approximable and singular numbers. We show that in higher dimensions this is not the case by proving that there exist continuum many Dirichlet improvable vectors that are neither badly approximable nor singular. This is a consequence of a stronger statement that involves very well approximable points. In the last section we formulate the notion of intermediate Dirichlet improvable sets concerning approximations by rational planes of every intermediate dimension and show that they coincide. This naturally extends a classical theorem of Davenport & Schmidt (1969) which states that the simultaneous form of Dirichlet’s theorem is improvable if and only if the dual form is improvable. Consequently, our main “continuum” result is equally valid for the corresponding intermediate Diophantine sets of badly approximable, singular and Dirichlet improvable points.

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© 2022 The Authors.

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