Before proceeding, we recall some notions and notations. An at most countable sequence
\(\{e_{i}\}_{i\in\mathcal{I} }\) in a separable Hilbert space
\(\mathcal{H}\) is called a
frame for
\(\mathcal{H}\) if there exist
\(0< C_{1}\le C_{2}<\infty\) such that
$$ C_{1}\|f\|^{2}\le\sum _{i \in\mathcal{I}} \bigl\vert \langle f, e_{i}\rangle \bigr\vert ^{2}\le C_{2}\|f\|^{2} \quad\mbox{for }f\in \mathcal{H}, $$
(1.1)
where
\(C_{1}\),
\(C_{2}\) are called
frame bounds; it is called a
Bessel sequence in
\(\mathcal{H}\) if the right-hand side inequality in (
1.1) holds, where
\(C_{2}\) is called a
Bessel bound. In particular,
\(\{e_{i}\}_{i\in\mathcal{I} }\) is called a
Parseval frame if
\(C_{1}=C_{2}=1\) in (
1.1). Given a frame
\(\{e_{i}\}_{i\in\mathcal{I} }\) for
\(\mathcal{H}\), a sequence
\(\{\tilde{e}_{i}\}_{i\in\mathcal{I} }\) is called a
dual of
\(\{e_{i}\}_{i\in\mathcal{I} }\) if it is a frame such that
$$ f= \sum_{i \in\mathcal{I}}\langle f, \tilde{e}_{i}\rangle e_{i}\quad\mbox{for } f\in\mathcal{H}. $$
(1.2)
It is easy to check that
\(\{e_{i}\}_{i\in\mathcal{I} }\) is also a dual of
\(\{\tilde{e}_{i}\}_{i\in\mathcal{I} }\) if
\(\{\tilde{e}_{i}\}_{i\in \mathcal{I} }\) is a dual of
\(\{e_{i}\}_{i\in\mathcal{I} }\). So, in this case, we say
\(\{ e_{i}\}_{i\in\mathcal{I} }\) and
\(\{\tilde{e}_{i}\}_{i\in\mathcal{I} }\) form a pair of dual frames for
\(\mathcal{H}\). It is well known that
\(\{e_{i}\}_{i\in\mathcal{I} }\) and
\(\{\tilde{e}_{i}\}_{i\in\mathcal{I} }\) form a pair of dual frames for
\(\mathcal{H}\) if they are Bessel sequences and satisfy (
1.2). The fundamentals of frames can be found in [
1‐
3]. The
Fourier transform of
\(c\in l^{2}(\mathbb {Z})\) is defined by
\(\hat{c}(\cdot)=\sum_{m\in \mathbb {Z}}c(m)e^{-2\pi im\cdot}\). For two sequences
c and
d on
\(\mathbb {Z}\), the
convolution
\(c*d\) is defined by
$$c*d(k)=\sum_{m\in \mathbb {Z}}c(k-m)d(m)\quad\mbox{for }k\in \mathbb {Z} $$
if it is well defined. The
Kronecker delta is defined by
\(\delta_{n, m} = \left \{ \scriptsize{\begin{array}{@{}l@{\quad}l} 1 &\mbox{if }n=m; \\ 0& \mbox{if }n\neq m. \end{array}} \right .\)
\(l_{0}(\mathbb {Z})\) denotes the set of finitely supported sequences on
\(\mathbb {Z}\). We denote by
I the identity operator on
\(l^{2}(\mathbb {Z})\), and by
\(\chi_{E}\) its characteristic function for a set
E. Write
\(\mathbb {R}_{+}=(0, \infty)\). For a positive number
\(a>1\), a function
h defined on
\(\mathbb {R}_{+}\) is said to be
a-dilation periodic if
\(h(a\cdot )=h(\cdot)\) on
\(\mathbb {R}_{+}\). For a function
f defined on
\([1, a)\), we define the function
f̃ on
\(\mathbb {R}_{+}\) by
$$\tilde{f}(\cdot)=f\bigl(a^{-l}\cdot\bigr)\quad\mbox{on } \bigl[a^{l}, a^{l+1}\bigr)\mbox{ for }l\in \mathbb {Z}, $$
which is called the
a-dilation periodization of
f. Obviously, it is
a-dilation periodic.
The translation operator
\(T_{x_{0}}\), the modulation operator
\(M_{x_{0}}\) with
\(x_{0}\in \mathbb {R}\), and the dilation
\(D_{c}\) with
\(c>0\) are, respectively, defined by
$$\begin{aligned}& T_{x_{0}}f(\cdot)=f(\cdot-x_{0}),\\& M_{x_{0}}f(\cdot)=e^{2\pi ix_{0}\cdot}f(\cdot), \end{aligned}$$
and
$$D_{c}f(\cdot)=\sqrt{c}f(c\cdot) $$
for
\(f\in L^{2}(\mathbb {R})\). They are the basis of wavelet analysis. Affine systems of the form
\(\{D_{a^{j}}T_{bk}\psi: j, k\in \mathbb {Z}\}\) with
\(\psi\in L^{2}(\mathbb {R})\) and
a,
\(b>0\), and Gabor systems of the form
\(\{E_{mb}T_{na}g: m, n\in \mathbb {Z}\}\) with
\(g\in L^{2}(\mathbb {R})\) and
a,
\(b>0\) have been extensively studied. However, dilation-and-modulation systems of the form
$$ \{M_{mb}D_{a^{j}}\psi: m, j\in \mathbb {Z} \} \quad\mbox{with }a, b>0 $$
(1.3)
have not been extensively studied. This paper focuses on the following systems that are like (
1.3):
$$ \{\widetilde{\psi_{m}}D_{a^{j}}\psi: m, j\in \mathbb {Z} \}\quad\mbox{with }a>0, $$
(1.4)
where
$$ \psi_{m}(\cdot)=\frac{1}{\sqrt{a-1}}e^{2\pi i\frac{m\cdot }{a-1}}\quad\mbox{on }[1, a)\mbox{ for }m\in \mathbb {Z}. $$
(1.5)
We will investigate the theory of
\(L^{2}(\mathbb {R}_{+})\)-frames of the form (
1.4). It is obvious that
\(L^{2}(\mathbb {R}_{+})\) can be considered as the Fourier transform of the Hardy space
\(H^{2}(\mathbb {R})\) defined by
$$H^{2}(\mathbb {R})=\bigl\{ f\in L^{2}(\mathbb {R}):\hat{f}(\cdot)=0 \mbox{ a.e. on } (-\infty, 0)\bigr\} , $$
where the Fourier transform is defined by
$$\hat{f}(\cdot)= \int_{\mathbb {R}}f(x)e^{-2\pi ix\cdot}\,dx\quad\mbox{for }f\in L^{1}(\mathbb {R})\cap L^{2}(\mathbb {R}) $$
and extended to
\(L^{2}(\mathbb {R})\) by the Plancherel theorem. Wavelet frames in
\(H^{2}(\mathbb {R})\) of the form
\(\{D_{a^{j}}T_{m}\varphi: j, m\in \mathbb {Z}\}\) were studied in [
4,
5], and some variations can be found in [
6‐
11]. By the Plancherel theorem, an
\(H^{2}(\mathbb {R})\)-frame
\(\{D_{2^{j}}T_{m}\varphi: j, m\in \mathbb {Z}\}\) leads to a
\(L^{2}(\mathbb {R}_{+})\)-frame
$$ \bigl\{ e^{-2\pi i2^{-j}m\cdot}\hat{\varphi}\bigl(2^{-j}\cdot \bigr): j, m\in \mathbb {Z} \bigr\} . $$
(1.6)
In (
1.6),
\(e^{-2\pi i2^{-j}m\cdot}\) is
\(2^{j}\mathbb {Z}\)-periodic with respect to additive operation, and the period depends on the dilation factor
\(2^{j}\). However,
\(\widetilde{\psi_{m}}\) in (
1.4) is
a-dilation periodic and unrelated to
j. Therefore, frames of the form (
1.4) are different from ones of the form (
1.6) for
\(L^{2}(\mathbb {R}_{+})\) and of independent interest. They are related to a kind of function-valued frames in [
12]. In [
13], numerical experiments were made to establish that the nonnegative integer shifts of the Gaussian function formed a Riesz sequence in
\(L^{2}(\mathbb {R}_{+})\). In [
14], a sufficient condition was obtained to determine whether the nonnegative translates form a Riesz sequence on
\(L^{2}(\mathbb {R}_{+})\).
The rest of this paper is organized as follows. Section
2 is devoted to characterizing frames and dual frames for
\(L^{2}(\mathbb {R}_{+})\) with the structure of (
1.4). Section
3 is devoted to Parseval frames and orthonormal bases for
\(L^{2}(\mathbb {R}_{+})\) of the form (
1.4). It turns out that Parseval frames, orthonormal bases, and orthonormal systems in
\(L^{2}(\mathbb {R}_{+})\) of the form (
1.4) are mutually equivalent to each other. It is worth noting that neither affine systems nor Gabor systems have such a property.