Derivation of connection between Rt and λ

[Liyubov]

We wanted to derive properly expression from initial code for connecting λ and Rt with @matteofrigo
In the code https://github.com/k-sys/covid-19 it is written that the derivation is beyond the scope of this notebook and the paper by Bettencourt is cited, but in the paper the derivation still lacks the proper estimate of the integral of k(t). Maybe someone had the same issues?
Thank you.