What is the lowest 2013 population estimate of south africa, which has a percentage greater than 19.8 and a 2011 population greater than 51,770,561?
CREATE TABLE "historical_data" (
  "rank" text,
  "province" text,
  "population_2011" real,
  "percentage" real,
  "population_estimate_2013" real
);
SELECT MIN("population_estimate_2013") FROM "historical_data" WHERE "percentage">19.8 AND "province"='south africa' AND "population_2011">'51,770,561';