https://dds.las.ac.cn/service/document?appId=a1vacbnPuv9qJRzJvdNXfsesj9JbIa6e&appKey=sPBA4vANLzjnW1k5eufEcc4k7dxfNraW&ts=1763013056120&signature=9e4195092cf3b3babc03d7408f5e8f60&articleTitle=Linear+Operators+and+Approximation+%2F+Lineare+Operatoren+und+Approximation&articleAuthor=P.+L.+Butzer%2C+J.-P.+Kahane%2C+B.+Sz%C3%B6kefalvi-Nagy&publicationTitle=Linear+Operators+and+Approximation+%2F+Lineare+Operatoren+und+Approximation&language=&publicationType=1&publishYear=1972&isn=9783034872850&publisher=Birkh%C3%A4user+Basel