@book { MentaleFitnessimTennis,
    title = "Mentale Fitness im Tennis",
    author = "Martin K. W. {Schweer}",
    year = "2021",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    doi = "10.3726/978-3-653-06229-8",
    url = "https://www.peterlang.com/document/1053625"
}