Geodesic
Positive integers $n$ such that $\sigma(\varphi(n))=\sigma(n)$
Journal of integer sequences, Tome 11 (2008) no. 1.

Voir la notice de l'article provenant de la source Electronic Library of Mathematics

Summary: In this paper, we investigate those positive integers $n$ for which the equality $\sigma (\phi (n)) = \sigma (n)$ holds, where $\sigma $ is the sum of the divisors function and $\phi $ is the Euler function.
Classification : 11A25, 11N37
Keywords: Euler function, sum of divisors 
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     author = {De Koninck, Jean-Marie and Luca, Florian},
     title = {Positive integers $n$ such that $\sigma(\varphi(n))=\sigma(n)$},
     journal = {Journal of integer sequences},
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     volume = {11},
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     year = {2008},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/JIS_2008__11_1_a5/}
}
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De Koninck, Jean-Marie; Luca, Florian. Positive integers $n$ such that $\sigma(\varphi(n))=\sigma(n)$. Journal of integer sequences, Tome 11 (2008) no. 1. http://geodesic.mathdoc.fr/item/JIS_2008__11_1_a5/