The core activity of the company Hirslanden cannot be directly determined based on the provided information, as the provided texts only contain error messages when connecting to the Hirslanden website and do not provide any information about the company's activities itself. However, based on general knowledge about the company Hirslanden, it can be said that it is a Swiss healthcare company specializing in the private hospital group. The core activity of the company Hirslanden is therefore likely to be the provision of medical services and the operation of hospitals.