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\bibitem{DiValentino_etal2016} Di Valentino, E., Gariazzo, S., Gerbino, M., Giusarma, E., \& Mena, O. (2016). Dark radiation and inflationary freedom after Planck 2015. \textit{Phys. Rev. D}, \textit{93}(8), 083523--28pp.

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