https://dds.las.ac.cn/service/document?appId=a1vacbnPuv9qJRzJvdNXfsesj9JbIa6e&appKey=sPBA4vANLzjnW1k5eufEcc4k7dxfNraW&ts=1763013340871&signature=71d89a6c3a195f472ee921c6cc4f7923&articleTitle=Differential+Equations&articleAuthor=D.+G.+de+Figueiredo%2C+C.+S.+H%C3%B6nig&publicationTitle=Differential+Equations&language=&publicationType=1&publishYear=1982&isn=9783540119517&publisher=Springer+Berlin+Heidelberg