The image of Carmichael's $\lambda$-function
  We show that the counting function of the set of values of the Carmichael $\lambda$-function is $x/(\log x)^{\eta+o(1)}$, where $\eta=1-(1+\log\log 2)/(\log 2)=0.08607...$. 
Kevin Ford, Florian Luca, Carl Pomerance
math.NT