@book { PortfolioAnalyseDarstellungAnwendungProbleme,
    title = "Portfolio-Analyse: Darstellung - Anwendung - Probleme",
    author = "René {Sauer}",
    year = "1997",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832403522",
    url = "https://m.diplom.de/document/216333"
}