W 0011/89 (Fibre fleece) vom 09.10.1989

To demonstrate that a group of inventions does not form a single inventive concept, an invitation to pay additional search fees pursuant to Article 17(3)(a) PCT must address the problem underlying the invention unless it is immediately clear that the technical details listed in the invitation cannot reasonably be seen as parts of an overall problem. If the invitation does not, when so required, address the problem set out in the application, it is not legally effective and any additional search fees must be refunded.