Plurimath

`doublestruck` (unary)

Doublestruck

AsciiMath

mathbb(x)

LaTeX

\mathbb{x}

MathML

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mstyle displaystyle="true">
    <mstyle mathvariant="double-struck">
      <mi>x</mi>
    </mstyle>
  </mstyle>
</math>

OMML

<m:oMathPara>
  <m:oMath>
    <m:r>
      <m:rPr>
        <m:scr m:val="double-struck"/>
      </m:rPr>
      <m:t>x</m:t>
    </m:r>
  </m:oMath>
</m:oMathPara>