## 1 Introduction  
Radical inventions (RIs) can boost a company's market position, by improving product competitiveness in the marketplace [1]. However, companies are more inclined to carry out incremental inventions that conform to a continual process of checking with optimization [2]. This is because the technological opportunities of incremental inventions can be directly identified from the problems of existing products [3], while the technological opportunities of RIs are manifested in the emergence of a distinctly different technology [4]. Hence, designers need to pay special attention to the identification of technological opportunities for RIs. Previous studies made attempts at combining the knowledge/technology of different domains [5, 6], creating markets [7], and developing criteria [8], to search for technological opportunities for RIs. However, these studies were mostly derived from innovation management strategies. Little is known about the effective identification of technological opportunities for RIs from technical information, and research based on patents for RIs is rarely discussed. Hence, further identification methods of technological opportunities for RIs need to be studied. Patents are considered one of the most reliable data resources for technological opportunity identification [9; 10; 11], due to their integrity of technical information, standardization of document management, and availability of data. Studies have suggested methods of identifying technological opportunities through drawing patent citation networks, to identify core patents as technological opportunities [12; 13; 14], establishing a morphological matrix to identify prioritized combinations of patents as technological opportunities [15], and constructing semantic patent maps, to identify patents hotspots and outlier patents as technological opportunities [16; 17; 18]. In recent years, with the development of natural language processing technology, innovative designs based on patent semantic information have received much attention [19; 20; 21]. However, the valuable information contained in the International Patent Classification (IPC) has been underestimated. The IPC is systematic and hierarchical, and the theme represented by each IPC symbol is mainly based on the function and structure of inventions, supplemented by the application of inventions [22]. Most previous studies have used the IPC as a patent classification/search tool, or further developed an IPC co-occurrence matrix, to measure technological distance [23]. Nevertheless, little effort has been made to investigate these features of the IPC system as a basis for developing a tool with which to identify technological opportunities. Having recognized these gaps, this paper proposes a method for identifying technological opportunities for RIs based on the IPC. For identifying technological opportunities for RIs, it is necessary to distinguish these from general technological opportunities based on the characteristics of RIs. RIs show a differentiated creativity, because they come from solving inventive problems using new technology. Thus, the value of difficulty (VOD) of the problems solved and the value of technological novelty (VON) of the new technology are applied to distinguish technological opportunities for RIs from others. Using IPC symbols and the order of filing dates of the patent set, technology manifested in a patent set with higher VONs is searched using a map of technological changes over time. Using the unstructured patent data, R&D themes of a patent set are decided by natural language processing and K-means clustering, and analyzed using complex network analysis to determine the VOD of each R&D theme. Finally, patents of technological opportunities for RIs are identified using an IPC-based coordinate system, based on VOC and VOD.