<div class="floatright"><a href="https://en.wikiversity.org/wiki/User:Watchduck/hat#Reverse_binary" title="The least significant bit in the binary numbers is on the left side!"><img alt="The least significant bit in the binary numbers is on the left side!" src="https://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Little-endian_binary.svg/100px-Little-endian_binary.svg.png" decoding="async" width="100" height="179" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Little-endian_binary.svg/150px-Little-endian_binary.svg.png 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Little-endian_binary.svg/200px-Little-endian_binary.svg.png 2x" data-file-width="819" data-file-height="1467"></a></div>
<p>Array of the <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\tbinom {8}{2}}=T_{7}=28}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="false" scriptlevel="0"><mrow><mrow class="MJX-TeXAtom-OPEN"><mo maxsize="1.2em" minsize="1.2em">(</mo></mrow><mfrac linethickness="0"><mn>8</mn><mn>2</mn></mfrac><mrow class="MJX-TeXAtom-CLOSE"><mo maxsize="1.2em" minsize="1.2em">)</mo></mrow></mrow></mstyle></mrow><mo>=</mo><msub><mi>T</mi><mrow class="MJX-TeXAtom-ORD"><mn>7</mn></mrow></msub><mo>=</mo><mn>28</mn></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle {\tbinom {8}{2}}=T_{7}=28}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/716895161be4b66e551c5eedc81dbe21a1b49eb0" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -1.005ex; width:13.885ex; height:3.343ex;" alt="{\displaystyle {\tbinom {8}{2}}=T_{7}=28}"></span> two-element <a href="https://en.wikipedia.org/wiki/Subset" class="extiw" title="w:Subset">subsets</a> of an 8-element set <small>(<a href="https://en.wikipedia.org/wiki/Binomial_coefficient" class="extiw" title="w:Binomial coefficient">Binomial coefficient</a>, <a href="https://en.wikipedia.org/wiki/Triangular_number" class="extiw" title="w:Triangular number">Triangular number</a>)</small>
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With the binary vectors interpreted as <a href="https://en.wikiversity.org/wiki/User:Watchduck/hat#Reverse_binary" class="extiw" title="v:User:Watchduck/hat">reverse binary numbers</a> this is <span style="white-space:nowrap"><a href="https://en.wikipedia.org/wiki/On-Line_Encyclopedia_of_Integer_Sequences" title="Sloane's"><img alt="Sloane's" src="https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/OEISicon_light.svg/11px-OEISicon_light.svg.png" decoding="async" width="11" height="15" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/OEISicon_light.svg/17px-OEISicon_light.svg.png 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/OEISicon_light.svg/22px-OEISicon_light.svg.png 2x" data-file-width="409" data-file-height="556"></a> <a href="//oeis.org/A018900" class="extiw" title="oeis:A018900">A018900</a></span> <a href="//oeis.org/A018900/table" class="extiw" title="oeis:A018900/table">as a square array</a>.
<table><tbody><tr><td style="padding: 15px;"><a href="//commons.wikimedia.org/wiki/File:Watchduck.svg" class="image"><img alt="Watchduck.svg" src="https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/Watchduck.svg/40px-Watchduck.svg.png" decoding="async" width="40" height="46" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/Watchduck.svg/60px-Watchduck.svg.png 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/Watchduck.svg/80px-Watchduck.svg.png 2x" data-file-width="703" data-file-height="806"></a>
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<td><a href="//commons.wikimedia.org/wiki/User:Watchduck" title="User:Watchduck">Watchduck</a><br><span style="color: gray; font-size: 80%;">You can name the author as "T. Piesk", "Tilman Piesk" or "Watchduck".</span>
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<span class="int-own-work" lang="en">Own work</span>