@book { MortgagedBackedSecurities,
    title = "Mortgaged Backed Securities",
    author = "Harold {Keller}",
    year = "2006",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832499327",
    url = "https://m.diplom.de/document/225049"
}