Coincidences in the values of the Euler and Carmichael functions
The Euler function has long been regarded as one of the most basic of the arithmetic functions. More recently, partly driven by the rise in importance of computational number theory, the Carmichael function has drawn an ever-increasing amount of attention. A large number of results have been obtained, both about the growth rate and about various arithmetical properties of the values of these two functions; see for example [2, 3, 5-7, 10-18, 20, 22, 23] and the references therein.
Acta Arith. 122 (2006) no.3, 207-234.