You can use AsciiMath or JLaTeXMath notation within PlantUML:

note right
Try also
<latex>P(y|\mathbf{x}) \mbox{ or } f(\mathbf{x})+\epsilon</latex>
end note


Bob -> Alice : Can you solve: <math>ax^2+bx+c=0</math>
Alice --> Bob: <math>x = (-b+-sqrt(b^2-4ac))/(2a)</math>

Standalone diagram

You can also use @startmath/@endmath to create standalone AsciiMath formula.

f(t)=(a_0)/2 + sum_(n=1)^ooa_ncos((npit)/L)+sum_(n=1)^oo b_n\ sin((npit)/L)

Or use @startlatex/@endlatex to create standalone JLaTeXMath formula.

\sum_{i=0}^{n-1} (a_i + b_i^2)

How is this working ?

To draw those formulas, PlantUML uses two OpenSource projects:

ASCIIMathTeXImg.js is small enough to be integrated into PlantUML standard distribution.

Since JLatexMath is bigger, you have to download it separately, then unzip the 4 jar files (batik-all-1.7.jar, jlatexmath-minimal-1.0.3.jar, jlm_cyrillic.jar and jlm_greek.jar) in the same folder as PlantUML.jar.