Let $f\in L^{1}(\mathbb T)$ and define the Fourier coefficient of $f$ : $\hat{f}(n)=\frac{1}{2\pi} \int \_{-\pi}^{\pi} f(t) e^{-int} dt; (n\in \mathbb Z)$ and we put,
 $$A(\mathbb T):= \{f\in L^{1}(\mathbb T): \hat{f}\in \ell^{1}(\mathbb Z), \ \text {that is,} \ \sum\_{n\in \mathbb Z} |\hat{f}(n)| < \infty \}.$$


Result of Katznelson: If $F$ is defined on on $[-1, 1]$ and composition of $F$ and $f$, $F(f) \in A(\mathbb T)$ whenever $f\in A(\mathbb T)$ and $f(\mathbb T)\subset [-1,1]$, then $F$ must be analytic on $[-1,1 ].$ 


As a corollary to this result, there exist $f\in A(\mathbb T)$ such that $|f|$ does not belong to $A(\mathbb T)$.


On the other hand,
Beurling has shown the the following:



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> **Result of Beurling**: If $f\in A(\mathbb T)$ such that $|\hat{f}(\pm n)| \leq c\_{n}, \ (n=0,1,2,...), $ where $c\_{n}\downarrow 0$ and $\sum\_{n=0}^{\infty}c\_{n} < \infty,$ then $|f|\in A(\mathbb T).$
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I read the above result in the book (by R. E. Edwards, Fourier series, A Modern Introduction, Volume-1; p.178); in which he state this result without proof, for further reading. I am unable to find the proper reference for the same, in web search. 


**My Request**: If possible, please, can you give me a proper reference book for the result of Beurling or in which paper of the Beurling this result has been appear ? (I guess this must be appear between the years 1955 and 1965)


Thanks a lot;



Result of Beurling concerning absolute convergence of Fourier series of |f|
This is Theorem V (page 16) from:


A. Beurling, [On the spectral synthesis of bounded functions](http://www.ams.org/mathscinet-getitem?mr=27891). Acta Math. 81 (1948).


In fact, Beurling proves the stronger statement:



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> > **Theorem** Let $f(x) = \sum\_{n=-\infty}^{\infty} a\_n e(nx)$ (with $a\_0=0$) have an absolutely convergent Fourier series such that $|a\_{\pm n}| \leq a\_n^{\*}$ where $a\_n^{\*}$ is a non-increasing sequence in $\ell^{1}$. Moreover, assume that $g(x) = \sum\_{n=-\infty}^{\infty} b\_n e(nx)$ (with $b\_0=0$) is a contraction of $f$. Then the Fourier series of $g(x)$ converges absolutely and $\sum\_{n=-\infty}^{\infty}|b\_n| \ll \sum\_{n=-\infty}^{\infty} a\_n^{\*}$.
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