## 1 Introduction
In this regard, research and development advocates digital twinning as one of several critical enablers. The novel technological applications that researchers envision in the context of digital twinning are just as ambitious. Medical research, for example, considers applications such as disease-trajectory estimation, optimization of medical-care timing, identification of biomarkers or elucidation of drug mechanisms, and patient-tailored prediction of treatment effects, employing digital twins of, e.g., a patient's immune system [8]. Given the potential for hazardous impacts of future digital-twinning applications on sensitive aspects of human well-being, the need to follow strict specifications on privacy, integrity, reliability, and safety is manifest. The upcoming 6G industry standard for communication technologies, which incorporates large parts of the technological infrastructure for the metaverse and other digital-twinning applications, summarizes such requirements by the term trustworthiness [9]. Depending on the potential hazards of an application, the physical entity's relevant properties must be reliably recoverable from the entity's digital twin. In practice, technological systems for critical applications must undergo technology assessment, which evaluates the implementation with regard to criteria of provable performance. When expressed in mathematical terms, e.g., by a margin of error, the recovered property must almost surely meet, such criteria entail \"sufficient\" and \"insufficient\" ways of representing a physical entity in virtual space. That is, the employed machine-readablelanguage must satisfy specific structural characteristics, such that the relevant properties can be reliably computed from any of the entity's machine-readable descriptions. We summarize this observation in terms of the following fundamental problem statement, which we aim to elaborate on throughout this article. Given an application that requires the processing of analog information, find a sufficient way to represent the information on the chosen hardware platform. The problem statement refers to general hardware platforms. As previously indicated, future virtual-twinning applications will not necessarily be limited to digital technology, c.f. [4]. However, in the scope of this article, we will only discuss traditional digital computing. Hence, in the problem statement, we may replace \"the chosen hardware platform\" with \"digital hardware\" in the context of the subsequent sections. So far, our discussion on the fundamental problem statement and the associated concepts has been abstract, without a clear picture of how they translate to actual engineering problems. Throughout the subsequent sections, we aim to draw a precise picture of machine-readable languages, relevant properties, and proper representations for signal-processing and communications-engineering applications that involve traditional SSI. Aside from the conceptual similarities between digital twinning and SSI we discussed above, SSI has relevant direct applications in digital twinning. In the context of general virtual twinning, ref. [10] discussed an actual implementation of such an application, c.f. Figure 1.  
Recall that traditional SSI aims at restoring the physical entity (i.e., a bandlimited signal) entirely. The relevant analytic result is known as (generalized) Plancherel-Polya Theorem, c.f. Section 2 and Figure 2: The bandlimited signal uniquely determines the corresponding sequence of samples, and vice versa. Accordingly, we expect that any property of the bandlimited signal should be recoverable from the sequence of sampling values. At this point, [PERSON]'s theory of digital computing enters the stage: A priori, the (generalized) Plancherel-Polya Theorem is a purely analytic result. For it to hold on the algorithmic level, effectiveness in the sense of computable analysis is required. In this context, we will analyze two machine-readable languages emerging from the (generalized) Plancherel-Polya Theorem for their structural properties.