...is L^1(\mathbb{R}).
The goal of the exercise is to prove that for some fixed enumeration of the rationals \lbrace q_n \rbrace_{n=1}^\infty, F(x) = \sum_{k=1}^\infty 2^{-k} f(x - q_k) ...is also L^1(\mathbb{R}). Proceed by simply defining the sequence of functions: F_n(x) = \sum_{k=1}^n 2^{-k} f(x - q_k) ...and first observing that F_n(x) is L^1(\mathbb{R}) \hspace{0.25cm} \forall n: \int_\mathbb{R} F_n(x) dx = \int_\mathbb{R} \sum_{k=1}^n 2^{-k} f(x - q_k) dx = \dots \ldots = \sum_{k=1}^n 2^{-k} \int_\mathbb{R} f(x - q_k)dx = C \sum_{k=1}^n 2^{-k} Where C = \int_\mathbb{R} f(x)dx (by translation invariance). Thus, since \lbrace F_n \rbrace_{n=1}^\infty is a collection of non-negative measurable functions such that F_n \leq F_{n+1} \hspace{0.25cm} \forall n, and F_n \nearrow F monotonically, we have by the Monotone Convergence Theorem, \int_\mathbb{R} |F(x)| dx = \int_\mathbb{R} F(x)dx = \lim_{n \to \infty} \int_\mathbb{R} F_n(x)dx = \ldots \ldots = C \sum_{k=1}^\infty 2^{-k} = C < \infty Thus, F \in L^1(\mathbb{R}). Next, observe that even if some function G = F almost everywhere, on any fixed interval I \subset \mathbb{R}, G|_I = F|_I almost everywhere. Thus, the sets: \mathcal{G}_I^n = \lbrace x \in G|_I \hspace{0.25cm} \big| \hspace{0.25cm} G(x) > n \rbrace \hspace{0.25cm} \text{and} \hspace{0.25cm} \mathcal{F}_I^n = \lbrace x \in F|_I \hspace{0.25cm} \big| \hspace{0.25cm} F(x) > n \rbrace Have the same (positive \forall n) measure. Thus, G must be unbounded on any interval.
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