rec-equivalent1

a is equivalent to not not a

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <apply>
    <csymbol cd="logic1">equivalent</csymbol>
    <ci>a</ci>
    <apply>
      <csymbol cd="logic1">not</csymbol>
      <apply>
        <csymbol cd="logic1">not</csymbol>
        <ci>a</ci>
      </apply>
    </apply>
  </apply>
</math>

Author: MathML 2 Recommendation