https://dds.las.ac.cn/service/document?appId=a1vacbnPuv9qJRzJvdNXfsesj9JbIa6e&appKey=sPBA4vANLzjnW1k5eufEcc4k7dxfNraW&ts=1763029740242&signature=684375105034a2c5694d8daf8aa391ad&articleTitle=Indefinite+Inner+Product+Spaces&articleAuthor=J%C3%A1nos+Bogn%C3%A1r&publicationTitle=Indefinite+Inner+Product+Spaces&language=&publicationType=1&publishYear=1974&isn=9783642655692&publisher=Springer+Berlin+Heidelberg