↑ “Earliest Uses of Symbols of Set Theory and Logic”. 9 tháng 1 năm 2010. Bản gốc lưu trữ 20/2/1999 (cập nhật 29/8/1998). Truy cập 16/11/2012. Wisely, we had decided to publish an installment establishing the system of notation for set theory, rather than wait for the detailed treatment that was to follow: it was high time to fix these notations once and for all, and indeed the ones we proposed, which introduced a number of modifications to the notations previously in use, met with general approval. Much later, my own part in these discussions earned me the respect of my daughter Nicolette, when she learned the symbol Ø for the empty set at school and I told her that I had been personally responsible for its adoption. The symbol came from the Norwegian alphabet, with which I alone among the Bourbaki group was familiar. Kiểm tra giá trị ngày tháng trong: |archivedate=, |accessdate= (trợ giúp)
