https://www.bloomberg.com/news/articles/2019-06-21/the-world-now-has-13-trillion-of-debt-with-below-zero-yields
https://borsen.dk/nyheder/oekonomi/artikel/l/383429/europas_ stoerste_oekonomi_ser_ind_i_en_verden_med_negativ_rente_paa_laan_ med_30_aars_loebetid.html
https://www.investing.com/news/economy-news/bankers-stunned-as-negative-rates-sweep-across-danish-mortgages-1877812
https://www.yardeni.com/pub/peacockfedecbassets.pdf
https://www.bis.org/publ/arpdf/ar2018e.pdf