<math> <!-- reference to OpenMath formal syntax definition of Bessel function --> <apply> <csymbol encoding="OpenMath" definitionURL="http://www.openmath.org/cd/BesselFunctions.ocd"> <msub><mi>J</mi><mn>0</mn></msub> </csymbol> <ci>y</ci> </apply> <!-- reference to human readable text description of Boltzmann's constant --> <csymbol encoding="text" definitionURL="www.example.org/universalconstants/Boltzmann.htm"> k </csymbol> </math>