 Cosine integral (source code)

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= Cosine integral
{title2=$\operatorname{Ci}(x)=-\int_x^\infty\cos t/t\,dt$}

For real $x>0$, its small-argument expansion is $\operatorname{Ci}(x)=\gamma_E+\log x+O(x^2)$. Integrating $\operatorname{Ci}'(x)=\cos x/x$ separates the logarithm from the regular correction. Its conventional constant is the <Euler--Mascheroni constant>; the primary series normalization is https://dlmf.nist.gov/6.6.E6 .

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