# What is the "natural" ($\natural$) symbol?

This paper talks about "$M^\natural$-concave functions". The paper defines what an M$^\natural$-concave function is, but, I would like to know specifically what does the $\natural$ symbol mean? Why is it called "natural" and where else in mathematics is it used?

• Sep 29 '16 at 5:55
• This post on TeX.SE implies that the OP was wishing to use it as a boundary connected sum in analogy to how $\sum$ is used for ordinary summation. Looking at other texts about boundary connected sums however, I see $\#$ or some other variation being used instead. Sep 29 '16 at 6:33
• Should we read for example M natural? May 27 '17 at 8:30
• It's the natural sign from music, to undo a sharp or flat set by the key signature. Unsure how it's used in math. Here's the Unicode for the symbol: fileformat.info/info/unicode/char/266e/index.htm Jul 4 '19 at 3:44