I am tracking Covid-19 in Germany and I need help figuring this out in a formula, I should have all the data needed to do the formulas but
I don't understand how to make the formulas according to this article tracking a virus at plus maths dot org
(sorry it won't let me post links)

1. Reproduction Rate R(t) explained in this document at blog dot covidactnow covid-risk-levels-matrics

Calculating Infection Growth Rate (Reproduction Rate R(t) )
To calculate the infection growth, a mathematical model combines trends in daily new cases from approximately the last 14 days, with estimates for other variables, such as how many days on average occur between infection and transmission.

I want to calculate it for the number of new infections and use nowcasting


2. Exponential Growth Rate, and growth rate per day or over time
I found something like this exponential growth or decay function is a function that grows or shrinks at a constant percent growth rate.
The equation can be written in the form f(x) = a(1 + r)x or f(x) = abx where b = 1 + r.
but not sure if this is correct I need help making an excel formula

here they talk about it at sciencedirect dot com
and at ipdjournal dot biomedcentral dot com

also I need a formula for rate of increase and rate of growth based on new cases daily



The second test I use to measure the growth is Incidence and Prevalence.
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3. Incidence Level - I have this formula and it works fine based on 7 day per 100k

4. Prevalence
see technologynetworks dot com immunology articles incidence-vs-prevalence-329073
How I calculated for it I took the # of active cases about 21 days, per 100k of population
I think this is right ?

But the guide says I need a threshold for the prevalence - to determine the risk, this I do not know how to calculate it


5. Having all of this data, and learning about nowcasting - how can I predict what the numbers could be in the next 7-14 days or longer ?
Should I use the growth rate ? or something else ? This place does it but I don't know how this link does it -- cardiopraxis dot de coronafuturum-corona-forecast-calculator-for-covid-19


6. Last sometimes my growth rates are a negative number based on the previous days number or the last 7 days total , I don't want the rates to go negative, what do you suggest ?

Link to my document won't work this forum won't let me attach it or link to it, but maybe is a way to email

Thanks in advance