Display:

$a_{\overline{n\hspace{.2em}|}\⁢i}$

Source:

<msub>
     <mi>a</mi>
     <mrow>
          <menclose notation="actuarial">
               <mi>n</mi>
          </menclose>
          <mo>&it;</mo>
          <mi>i</mi>
     </mrow>
</msub>