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Multipliers of spaces of derivatives

Jan Mařík, Clifford E. Weil (2004)

Mathematica Bohemica

For subspaces, X and Y , of the space, D , of all derivatives M ( X , Y ) denotes the set of all g D such that f g Y for all f X . Subspaces of D are defined depending on a parameter p [ 0 , ] . In Section 6, M ( X , D ) is determined for each of these subspaces and in Section 7, M ( X , Y ) is found for X and Y any of these subspaces. In Section 3, M ( X , D ) is determined for other spaces of functions on [ 0 , 1 ] related to continuity and higher order differentiation.

Normal numbers and subsets of N with given densities

Haseo Ki, Tom Linton (1994)

Fundamenta Mathematicae

For X ⊆ [0,1], let D X denote the collection of subsets of ℕ whose densities lie in X. Given the exact location of X in the Borel or difference hierarchy, we exhibit the exact location of D X . For α ≥ 3, X is properly D ξ ( Π α 0 ) iff D X is properly D ξ ( Π 1 + α 0 ) . We also show that for every nonempty set X ⊆[0,1], D X is Π 3 0 -hard. For each nonempty Π 2 0 set X ⊆ [0,1], in particular for X = x, D X is Π 3 0 -complete. For each n ≥ 2, the collection of real numbers that are normal or simply normal to base n is Π 3 0 -complete. Moreover, D , the...

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