You are looking for the symbol U+225C. It is not contained in OpenSymbol. So first you need to find a font, which contains a glyph for this character. I've got more than 10 fonts in my font collection, which contain a glyph for U+225C. If you will publish the text in web too, you might go with one of the MathJax fonts, if you are on Windows "Segoe UI Symbol" might work for you. Other candidates are "Lucida Sans Unicode", "STIX", or "Arial Unicode MS".

Next step is to add this character to the Math catalog, the Symbol Set. See chapter "Catalog customization" in the Math guide tells you how to do that. It gets the name Ux225C for example. https://wiki.documentfoundation.org/i...

If you want to generate MathML and generate a `<mo>`

-element, then you should use it as operator, e.g `(f+g)(x) boper %Ux225C f(x)+g(x)`

.