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  <title>Condense — mod 9</title>
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  <h1>Closed-form digital root</h1>
  <p><code>a</code> is a Number; <code>b</code> is eighteen nines as <code>BigInt</code>. Then <strong>Run</strong>.</p>
  <pre id=out></pre>
  <script>!function(){var n=999999999999999999n,r=[];r.push("dr(9875) = "+function(n){if(0===n)return 0;var r=n%9;return 0===r?9:r}(9875)+" (closed form)"),r.push("dr("+n+") = "+function(n){if(0n===n)return 0;var r=n%9n;return 0n===r?9:Number(r)}(n)+" (closed form)"),document.getElementById("out").textContent=r.join(String.fromCharCode(10))}()</script>
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JavaScript Condense a Number — closed form + BigInt (Try It)