https://dds.las.ac.cn/service/document?appId=a1vacbnPuv9qJRzJvdNXfsesj9JbIa6e&appKey=sPBA4vANLzjnW1k5eufEcc4k7dxfNraW&ts=1763013060593&signature=432dd76c8256d4e1bb909b19990d3bd2&articleTitle=Linear+Operators+and+Approximation+%2F+Lineare+Operatoren+und+Approximation&articleAuthor=Bautzer%2C++Kahane%2C++Nagy&publicationTitle=Linear+Operators+and+Approximation+%2F+Lineare+Operatoren+und+Approximation&language=&publicationType=1&publishYear=1972&isn=9783764305901&publisher=Birkh%C3%A4user+Basel