@book { RelationshipUTurnApproachestoIncreasetheValueofanUnprofitableCustomer,
    title = "Relationship U-Turn: Approaches to Increase the Value of an Unprofitable Customer",
    author = "Kerstin {Helmes}",
    year = "2011",
    publisher = "BACHELOR + MASTER PUBLISHING",
    address = "Hamburg, Deutschland",
    url = "https://m.bachelor-master-publishing.de/document/296433"
}