Fig 1.
Daily reported number of COVID-19 new positive cases of Japan from 16 January 2020 to 21 February 2023.
(a) Original data, [inset] enlargement of the original data from 16 January 2020 to 30 June 2021. (a’) Histogram of the original data. (b) Logarithm-transformed data of the original data (solid line) and its optimum LSF curve (dashed line). (c) MEM-PSD of the residual data in the low-frequency range (f ≤ 1.2). (d) Residual data obtained by subtracting the LSF curve from the log-data. (d’) Histogram of the residual data. Dashed vertical line in d indicates the boundary of phase I (pre-Tokyo Olympic Games, 11 February 2020–22 July 2021) and phase II (post-Tokyo Olympic Games, 23 July 2021–21 February 2023).
Fig 2.
MEM-PSDs for two ranges of the residual data (f ≤ 50.0).
(a) Phase I (pre-Tokyo Olympic Games, 11 February 2020–22 July 2021). (b) Phase II (post-Tokyo Olympic Games, 23 July 2021–21 February 2023). In each figure, the blue line in the low-frequency range (f ≤ 12.0) indicates the regression line with gradient λ of Eq (4). See the text for details.
Fig 3.
Comparison of the least squares fitting curve (solid line) with the residual data (red dot).
(a) Phase I (pre-Tokyo Olympic Games, 11 February 2020–22 July 2021). (b) Phase II (post- Tokyo Olympic Games, 23 July 2021–21 February 2023).
Table 1.
Frequencies and powers of the three dominant spectral peaks shown in Fig 2A and 2b.
(a) Phase I (pre-Tokyo Olympic Games, 11 February 2020–22 July 2021). (b) Phase II (post- Tokyo Olympic Games, 23 July 2021–21 February 2023).
Fig 4.
Close-up of the low-frequency region (f ≤ 6.0) in Fig 2A and 2B.
(a) Phase I (pre-Tokyo Olympic Games, 11 February 2020–22 July 2021). (b) Phase II (post- Tokyo Olympic Games, 23 July 2021–21 February 2023).
Fig 5.
Three-dimensional spectral array for the residual data in the frequency range of 1.0 ≤ f ≤ 10.0.
Fig 6.
Temporal variations of the frequencies of dominant spectral peaks detected in the frequency range of 1.0 ≤ f ≤ 4.0.
The three red dashed lines indicate f = 3.0 before May 2021, f = 2.2 from July to November 2021, and f = 2.0 thereafter.