@book { ElSistemaPolticodelosEEUU,
    title = "El Sistema Político de los EEUU",
    author = "Elfi Victoria {Siebert}",
    year = "2004",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783638307055",
    url = "https://www.hausarbeiten.de/document/29089"
}