On the page:
https://en.wikipedia.org/wiki/Carmichael_number
at the part "distribution", lower and upper bounds for the number of Carmichael numbers below $x$ (denoted by $C(x)$) are given. Two questions about these bounds :
Which constant can be used for the upper bound ? Does $k_2=1$ do the job ?
What does "sufficiently large" mean for the lower bound ? For which $x$ does $\large C(x)>x^\frac{2}{7}$ hold ?