In the different countries of the eurozone there can be only one nominal interest rate. However with divergent inflation rates in different countries real interest rates are also divergent. Hence, different growth and inflation rates. A competitivity difference of between 15 and 20% has appeared between countries like Greece, Spain and Ireland on hand and Germany on the other.
AFTALION, F. (2010). L'euro : le vice de construction. Le Monde Initiatives.