Geodesic
Positive integers \(n\) such that \(\sigma(\varphi(n))=\sigma(n)\)
Journal of integer sequences, Tome 11 (2008) no. 1
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 
@article{JIS_2008__11_1_a5,
     author = {De Koninck,  Jean-Marie and Luca,  Florian},
     title = {Positive integers \(n\) such that \(\sigma(\varphi(n))=\sigma(n)\)},
     journal = {Journal of integer sequences},
     year = {2008},
     volume = {11},
     number = {1},
     zbl = {1188.11048},
     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/