Table A.7.

Part 2 of the list of symbols and notations used in this paper.

Symbol	Description
$p$	number of autoregressive terms in ARIMAX
$d$	number of nonseasonal differences needed for stationarity in ARIMAX
$q$	number of lagged forecast errors in ARIMAX
$\eta$	constant of the ARIMAX
${\varphi }_i$	$i$-th element of parameter $\varphi$ in ARIMAX, for $i=1,\dots ,p$
${\theta }_j$	$j$-th element of parameter $\theta$ in ARIMAX, for $j=1,\dots q$
${\zeta }_l$	$l$-th element of parameter $\zeta$ in ARIMAX, for $l=1,\dots ,n$
$e_{t-j}$	error terms of the ARIMAX, for $j=1,\dots ,q$
$\mathit{nc}$	number of days in the dataset
$c_i^t$	observed number of COVID-19 cases on day $t$ in city $i$
${\widehat{c}}_i^t$	predicted number of COVID-19 cases on day $t$ in city $i$
$e_i^t$	error in the prediction of the number of COVID-19 cases on day $t$ in city $i$
$s_i^t$	absolute value of the difference between 1 and $\frac{c_i^t}{c_i^{(t-1)}}$
$S^t$	set $S^t=\{s_i^t,\forall i\}$
${\widehat{S}}^t$	spectrum of $S^t$
${\lambda }_t$	eigenvalues of graph Laplacian
$R^t$	time series obtained by applying the inverse Fourier transform in ${\widehat{R}}^t$
$r_i^t$	$i$-th element of vector $R^t$