Carmichael numbers are composite $n$ for which $$a^{n-1}\equiv1\quad(mod\ n)$$ is true for every prime $a<n$.
Part of a proof I'm currently working through includes the condition that for Carmichael numbers $n$, $(p_i -1)\mid(n-1)$ is true for every prime factor $p_i$.
I've tested this for some Carmichael numbers and it is true while for non-Carmichael numbers it not always appears to hold. I just don't quite understand why this is true.