= Solution
Assume the <infectivity profile> is separable:
$$
\beta_{t,\tau}=R_tg_\tau,
\qquad g_\tau\geq0,
\qquad \sum_{\tau\geq1}g_\tau=1.
$$
Then $g_\tau$ is the discretized <generation-interval distribution>, $R_t$ is the <instantaneous reproduction number>, and the <infectious disease renewal equation> becomes
$$
\Delta_t=R_t\Lambda_t,
\qquad
\Lambda_t=\sum_{\tau=1}^{t}g_\tau\Delta_{t-\tau}.
$$
Hence $R_t=\Delta_t/\Lambda_t$ whenever the total infectiousness $\Lambda_t$ is positive.
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