
Let L 1 {L^1} be the space of all complex-valued 2 π 2\pi -periodic integrable functions f f and let L 1 ^ \widehat {{L^1}} be the space of sequences of Fourier coefficients f ^ \hat f . A sequence λ \lambda is an ( L 1 ^ → L 1 ^ ) \left ( {\widehat {{L^1}} \to \widehat {{L^1}}} \right ) multiplier if λ ⋅ f ^ = ( λ ( n ) f ^ ( n ) ) \lambda \cdot \hat f = \left ( {\lambda \left ( n \right )\hat f\left ( n \right )} \right ) belongs to L 1 ^ \widehat {{L^1}} for every f f in L 1 {L^1} . The space of even sequences of bounded variation is defined by b υ = { λ | λ n = λ − n , ∑ k = 0 ∞ | Δ λ k | + sup n | λ n | > ∞ } b\upsilon = \left \{ {\lambda \left | {{\lambda _n} = {\lambda _{ - n}},\sum \nolimits _{k = 0}^\infty {\left | {\Delta {\lambda _k}} \right |} + {{\sup }_n}\left | {{\lambda _n}} \right | > \infty } \right .} \right \} , where Δ λ k = λ k − λ k + 1 \Delta {\lambda _k} = {\lambda _k} - {\lambda _{k + 1}} and the space of even bounded quasiconvex sequences is defined by q = { λ | λ n = λ − n , ∑ k = 1 ∞ k | Δ 2 λ k | + sup n | λ n | > ∞ } q = \left \{ {\lambda \left | {{\lambda _n} = {\lambda _{ - n}},\sum \nolimits _{k = 1}^\infty {k\left | {{\Delta ^2}{\lambda _k}} \right | + {{\sup }_n}\left | {{\lambda _n}} \right | > \infty } } \right .} \right \} , where Δ 2 λ k = Δ λ k − Δ λ k + 1 {\Delta ^2}{\lambda _k} = \Delta {\lambda _k} - \Delta {\lambda _{k + 1}} . It is well known that q ⊂ b υ q \subset b\upsilon and q ⊂ ( L 1 ^ → L 1 ^ ) q \subset \left ( {\widehat {{L^1}} \to \widehat {{L^1}}} \right ) but b υ ⊄ ( L 1 ^ → L 1 ^ ) b\upsilon \not \subset \left ( {\widehat {{L^1}} \to \widehat {{L^1}}} \right ) . This result is significantly improved by finding an increasing family of sequence spaces d υ p d{\upsilon _p} between q q and b υ b\upsilon which are ( L 1 ^ → L 1 ^ ) \left ( {\widehat {{L^1}} \to \widehat {{L^1}}} \right ) multipliers. Since the ( L 1 ^ → L 1 ^ ) \left ( {\widehat {{L^1}} \to \widehat {{L^1}}} \right ) multipliers are the 2 π 2\pi -periodic measures, this result gives sufficient conditions for a sequence to be the Fourier coefficients of a measure.
Fourier coefficients, Fourier series of functions with special properties, special Fourier series, Convolution, factorization for one variable harmonic analysis, multipliers, Fourier coefficients, sequence spaces, Multipliers in one variable harmonic analysis
Fourier coefficients, Fourier series of functions with special properties, special Fourier series, Convolution, factorization for one variable harmonic analysis, multipliers, Fourier coefficients, sequence spaces, Multipliers in one variable harmonic analysis
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