s="strut" style="height:0.85396em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.05724em;">j</span></span></span></span> such that <span class="katex"><span class="katex-mathml"><math><semantics><mrow><mi>i</mi><mo>+</mo><mi>j</mi></mrow><annotation encoding="application/x-tex">i + j</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.74285em;vertical-align:-0.08333em;"></span><span c