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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Sparse network</span></span>
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<div id="mw-content-text" class="mw-body-content mw-content-ltr" lang="en" dir="ltr"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><p>In <a href="Network_science" title="Network science">network science</a>, a <b>sparse network</b> has <i>much fewer</i> links than the possible maximum number of links within that network (the opposite is a <b>dense network</b>). The study of sparse networks is a relatively new area primarily stimulated by the study of real networks, such as social and computer networks.<sup id="cite_ref-:0_1-0" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
</p><p>The notion of <i>much fewer</i> links is, of course, colloquial and informal. While a threshold for a particular network may be invented, there is no universal threshold that defines what <i>much fewer</i> actually means. As a result, there is no formal sense of sparsity for any finite network, despite widespread agreement that most empirical networks are indeed sparse. There is, however, a formal sense of sparsity in the case of infinite network models, determined by the behavior of the number of edges (M) and/or the average degree (<span class="nowrap">⟨k⟩</span>) as the number of nodes (N) goes to infinity.<sup id="cite_ref-newman_2-0" class="reference"><a href="#cite_note-newman-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Definitions">Definitions</h2></div>
<p>A simple unweighted network of size <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle N}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>N</mi>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle N}</annotation>
</semantics>
</math></span><img src="./f5e3890c981ae85503089652feb48b191b57aae3.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:2.064ex; height:2.176ex;" alt="{\displaystyle N}" loading="lazy"></span> is called sparse if the number of links <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle M}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>M</mi>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle M}</annotation>
</semantics>
</math></span><img src="./f82cade9898ced02fdd08712e5f0c0151758a0dd.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:2.442ex; height:2.176ex;" alt="{\displaystyle M}" loading="lazy"></span> in it is much smaller than the maximum possible number of links <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle M_{max}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>m</mi>
<mi>a</mi>
<mi>x</mi>
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</msub>
</mstyle>
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<annotation encoding="application/x-tex">{\displaystyle M_{max}}</annotation>
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</math></span><img src="./c60c6220c155da2b6486fd364309ae961fe588d6.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:5.739ex; height:2.509ex;" alt="{\displaystyle M_{max}}" loading="lazy"></span>:<sup id="cite_ref-:0_1-1" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle M\ll M_{max}={N \choose 2}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>M</mi>
<mo>≪<!-- ≪ --></mo>
<msub>
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>m</mi>
<mi>a</mi>
<mi>x</mi>
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</msub>
<mo>=</mo>
<mrow class="MJX-TeXAtom-ORD">
<mrow>
<mrow class="MJX-TeXAtom-OPEN">
<mo maxsize="2.047em" minsize="2.047em">(</mo>
</mrow>
<mfrac linethickness="0">
<mi>N</mi>
<mn>2</mn>
</mfrac>
<mrow class="MJX-TeXAtom-CLOSE">
<mo maxsize="2.047em" minsize="2.047em">)</mo>
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</mstyle>
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<annotation encoding="application/x-tex">{\displaystyle M\ll M_{max}={N \choose 2}}</annotation>
</semantics>
</math></span><img src="./ee31f0eb25b1ece080fa522ea70f13383c8c3936.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.505ex; width:20.378ex; height:6.176ex;" alt="{\displaystyle M\ll M_{max}={N \choose 2}}" loading="lazy"></span> .
</p><p>In any given (real) network, the number of nodes <i>N</i> and links <i>M</i> are just two numbers, therefore the meaning of the <i>much smaller</i> sign (<span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \ll }">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mo>≪<!-- ≪ --></mo>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle \ll }</annotation>
</semantics>
</math></span><img src="./10563f31a4178c71bef006ea5c601e908f984136.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:2.324ex; height:1.843ex;" alt="{\displaystyle \ll }" loading="lazy"></span> above) is purely colloquial and informal, and so are statements like "many real networks are sparse."
</p><p>However, if we deal with a synthetic graph sequence <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span>, or a network model that is well defined for networks <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> of any size <i>N</i> = 1,2,...,<span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \infty }">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi mathvariant="normal">∞<!-- ∞ --></mi>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle \infty }</annotation>
</semantics>
</math></span><img src="./c26c105004f30c27aa7c2a9c601550a4183b1f21.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:2.324ex; height:1.676ex;" alt="{\displaystyle \infty }" loading="lazy"></span>, then the <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \ll }">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mo>≪<!-- ≪ --></mo>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle \ll }</annotation>
</semantics>
</math></span><img src="./10563f31a4178c71bef006ea5c601e908f984136.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:2.324ex; height:1.843ex;" alt="{\displaystyle \ll }" loading="lazy"></span> attains its usual formal meaning:
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle M\ll M_{max}\iff M=o(M_{max})\iff \lim _{N\rightarrow \infty }{\frac {M}{M_{max}}}=0}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>M</mi>
<mo>≪<!-- ≪ --></mo>
<msub>
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>m</mi>
<mi>a</mi>
<mi>x</mi>
</mrow>
</msub>
<mspace width="thickmathspace"></mspace>
<mo stretchy="false">⟺<!-- ⟺ --></mo>
<mspace width="thickmathspace"></mspace>
<mi>M</mi>
<mo>=</mo>
<mi>o</mi>
<mo stretchy="false">(</mo>
<msub>
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>m</mi>
<mi>a</mi>
<mi>x</mi>
</mrow>
</msub>
<mo stretchy="false">)</mo>
<mspace width="thickmathspace"></mspace>
<mo stretchy="false">⟺<!-- ⟺ --></mo>
<mspace width="thickmathspace"></mspace>
<munder>
<mo movablelimits="true" form="prefix">lim</mo>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
<mo stretchy="false">→<!-- → --></mo>
<mi mathvariant="normal">∞<!-- ∞ --></mi>
</mrow>
</munder>
<mrow class="MJX-TeXAtom-ORD">
<mfrac>
<mi>M</mi>
<msub>
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>m</mi>
<mi>a</mi>
<mi>x</mi>
</mrow>
</msub>
</mfrac>
</mrow>
<mo>=</mo>
<mn>0</mn>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle M\ll M_{max}\iff M=o(M_{max})\iff \lim _{N\rightarrow \infty }{\frac {M}{M_{max}}}=0}</annotation>
</semantics>
</math></span><img src="./4cb2bb65e8c2617252973b2636dbf66596a13190.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; width:55.775ex; height:5.676ex;" alt="{\displaystyle M\ll M_{max}\iff M=o(M_{max})\iff \lim _{N\rightarrow \infty }{\frac {M}{M_{max}}}=0}" loading="lazy"></span>.
</p><p>In other words, a network sequence or model <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> is called <i>dense</i> or <i>sparse</i> depending on whether the (expected) average degree <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \langle k\rangle =2M/N}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo>
<mi>k</mi>
<mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo>
<mo>=</mo>
<mn>2</mn>
<mi>M</mi>
<mrow class="MJX-TeXAtom-ORD">
<mo>/</mo>
</mrow>
<mi>N</mi>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle \langle k\rangle =2M/N}</annotation>
</semantics>
</math></span><img src="./65f4de99fff8314be2a279d223e551ba3b16db0b.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:12.95ex; height:2.843ex;" alt="{\displaystyle \langle k\rangle =2M/N}" loading="lazy"></span> in <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> scales <i>linearly</i> or <i>sublinearly</i> with <i>N</i>:<sup id="cite_ref-newman_2-1" class="reference"><a href="#cite_note-newman-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-bela_3-0" class="reference"><a href="#cite_note-bela-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> is <i>dense</i> if <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \langle k\rangle =O(N)}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo>
<mi>k</mi>
<mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo>
<mo>=</mo>
<mi>O</mi>
<mo stretchy="false">(</mo>
<mi>N</mi>
<mo stretchy="false">)</mo>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle \langle k\rangle =O(N)}</annotation>
</semantics>
</math></span><img src="./e7e334fd73307f4e1def01b6bc7525521cb328a4.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:11.765ex; height:2.843ex;" alt="{\displaystyle \langle k\rangle =O(N)}" loading="lazy"></span>;
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> is <i>sparse</i> if <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \langle k\rangle =o(N)}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo>
<mi>k</mi>
<mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo>
<mo>=</mo>
<mi>o</mi>
<mo stretchy="false">(</mo>
<mi>N</mi>
<mo stretchy="false">)</mo>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle \langle k\rangle =o(N)}</annotation>
</semantics>
</math></span><img src="./de678b0b0810919cfb9b9684e040b71764d95a8a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:11.12ex; height:2.843ex;" alt="{\displaystyle \langle k\rangle =o(N)}" loading="lazy"></span>.
</p><p>An important subclass of sparse networks are networks whose average degree is either constant or converges to a constant. Some authors call only such networks sparse, while others reserve special names for them: <sup id="cite_ref-janson_4-0" class="reference"><a href="#cite_note-janson-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
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</msub>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> is <i>truly sparse</i> or <i>extremely sparse</i> or <i>ultrasparse</i> if <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \langle k\rangle =O(1)}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo>
<mi>k</mi>
<mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo>
<mo>=</mo>
<mi>O</mi>
<mo stretchy="false">(</mo>
<mn>1</mn>
<mo stretchy="false">)</mo>
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</mrow>
<annotation encoding="application/x-tex">{\displaystyle \langle k\rangle =O(1)}</annotation>
</semantics>
</math></span><img src="./f2c9921d5f99ada84b60e4188e459c61550982bb.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:10.864ex; height:2.843ex;" alt="{\displaystyle \langle k\rangle =O(1)}" loading="lazy"></span>.
</p><p>There also exist alternative, stricter definitions of network sparsity requiring the convergence of the degree distribution in <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle G_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>G</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
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</msub>
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<annotation encoding="application/x-tex">{\displaystyle G_{N}}</annotation>
</semantics>
</math></span><img src="./e784915eb57f908dc0064588f9d41ab93ff5bbaa.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.518ex; height:2.509ex;" alt="{\displaystyle G_{N}}" loading="lazy"></span> to a well defined limit at <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle N\rightarrow \infty }">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>N</mi>
<mo stretchy="false">→<!-- → --></mo>
<mi mathvariant="normal">∞<!-- ∞ --></mi>
</mstyle>
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<annotation encoding="application/x-tex">{\displaystyle N\rightarrow \infty }</annotation>
</semantics>
</math></span><img src="./3c6337428441dbe11ca917f0d9115e8cf3a78f40.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:8.001ex; height:2.176ex;" alt="{\displaystyle N\rightarrow \infty }" loading="lazy"></span>.<sup id="cite_ref-remco_5-0" class="reference"><a href="#cite_note-remco-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> According to this definition, the <a href="Star_(graph_theory)" title="Star (graph theory)">N-star graph</a> <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S_{N}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<msub>
<mi>S</mi>
<mrow class="MJX-TeXAtom-ORD">
<mi>N</mi>
</mrow>
</msub>
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</mrow>
<annotation encoding="application/x-tex">{\displaystyle S_{N}}</annotation>
</semantics>
</math></span><img src="./aca805f5a6c6548a825d119230ab751152321ff1.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.116ex; height:2.509ex;" alt="{\displaystyle S_{N}}" loading="lazy"></span>, for example, is not sparse.
</p>
<div class="mw-heading mw-heading2"><h2 id="Node_degree_distribution">Node degree distribution</h2></div>
<p>The node degree distribution changes with the increasing connectivity. Different link densities in the complex networks have different node-degree distribution, as Flickr Network Analysis suggests.<sup id="cite_ref-Scholz2015_6-0" class="reference"><a href="#cite_note-Scholz2015-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> The sparsely connected networks have a scale free, <a href="Power_law" title="Power law">power law</a> distribution. With increasing connectivity, the networks show increasing divergence from power law. One of the main factors, influencing on the network connectivity is the <a href="Node_(networking)" title="Node (networking)">node similarity</a>. For instance, in <a href="Social_networks" class="mw-redirect" title="Social networks">social networks</a>, people are likely to be linked to each other if they share common social background, interests, tastes, beliefs, etc. In context of biological networks, proteins or other molecules are linked if they have exact or complementary fit of their complex surfaces.<sup id="cite_ref-Scholz2015_6-1" class="reference"><a href="#cite_note-Scholz2015-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading2"><h2 id="Common_terminology">Common terminology</h2></div>
<p>If the nodes in the networks are not weighted, the structural components of the network can be shown through <a href="Adjacency_matrix" title="Adjacency matrix">adjacency matrix</a>. If the most elements in the matrix are zero, such matrix is referred as <a href="Sparse_matrix" title="Sparse matrix">sparse matrix</a>. In contrast, if most of the elements are nonzero, then the matrix is <a href="Dense_matrix" class="mw-redirect" title="Dense matrix">dense</a>. The sparsity or density of the matrix is identified by the fraction of the zero element to the total number of the elements in the matrix. Similarly, in the context of <a href="Graph_theory" title="Graph theory">graph theory</a>, if the number of links is close to its maximum, then the graph would be known as <a href="Dense_graph" title="Dense graph">dense graph</a>. If the number of links is lower than the maximum number of links, this type of graphs are referred as <a href="Sparse_graph" class="mw-redirect" title="Sparse graph">sparse graph</a>.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading2"><h2 id="Applications">Applications</h2></div>
<p>Sparse Network can be found in <a href="Social_network" title="Social network">social</a>, <a href="Computer_network" title="Computer network">computer</a> and <a href="Biological_network" title="Biological network">biological networks</a>, as well as, its applications can be found in <a href="Transport_network" class="mw-redirect" title="Transport network">transportation</a>, power-line, citation networks, etc. Since most real networks are large and sparse, there were several models developed to understand and analyze them.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> These networks have inspired sparse <a href="Network_on_a_chip" title="Network on a chip">network-on-chip</a> design in multiprocessor embedded <a href="Computer_engineering" title="Computer engineering">computer engineering</a>.
</p><p>Sparse networks also induce cheaper computations by making it efficient to store the network as an <a href="Adjacency_list" title="Adjacency list">Adjacency list</a>, rather than an <a href="Adjacency_matrix" title="Adjacency matrix">Adjacency matrix</a>. For example, when using an adjacency list, iterating over a node's neighbors can be achieved in O(M/N), whereas it is achieved in O(N) with an adjacency matrix.<sup id="cite_ref-newman_2-2" class="reference"><a href="#cite_note-newman-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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