the alternating sequence <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 a_{n}=(-1)^{n}}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><msub><mi>a</mi><mrow class="MJX-TeXAtom-ORD"><mi>n</mi></mrow></msub><mo>=</mo><mo stretchy="false">(</mo><mo>−<!-- − --></mo><mn>1</mn><msup><mo stretchy="false">)</mo><mrow class="MJX-TeXAtom-ORD"><mi>n</mi></mrow></msup></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle a_{n}=(-1)^{n}}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/d28e5e8a3c70cf22978a7407aac090da6d914cdf" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.838ex; width:11.545ex; height:2.843ex;" alt="a_{n}=(-1)^{n}"></span>
<a href="//commons.wikimedia.org/wiki/User:Stephan_Kulla" title="User:Stephan Kulla">Stephan Kulla</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>