Many sources assert that China appears to have eliminated COVID-19 within the country, except for the odd imported case or cluster, which is quickly isolated and eliminated. For instance, Worldometer's coronavirus tracker, based on data provided by each country's government, shows no new cases fo...

Background So the aim is to find the value of the series (usually found in Feynman diagrams): $$ S= a_0 + 1!a_1 \epsilon + 2!a_2 \epsilon^2 + \dots + n!a_n \epsilon^n $$ where $\epsilon \to 0$. We start by considering the function: $$ f = \prod_{r=1}^N ( r\epsilon)$$ To make sense of we take log ...

Suppose that $A(g)$ is given by some perturbative expansion around $g=0$: $$A(g) = \sum_n c_n g^n.$$ Then the statement that this expansion is asymptotic means that the radius of convergence is zero: for fixed $g$, no matter how small, the limit $$\lim_{N \to \infty} \; \sum_{n < N} c_n g^n$$ div...