Alwin Korselt

Alwin Korselt is associated with Korselt criterion for Carmichael numbers. The criterion characterizes a composite positive integer $n$ by squarefreeness and the divisibilities $p-1\mid n-1$ for every prime divisor $p$ of $n$.

It turns a universal modular-exponent condition into a finite collection of arithmetic tests on prime factors. For example, $561=3\cdot 11\cdot 17$ is squarefree, and $2,10,16$ all divide $560$, giving the standard example of a Carmichael number.^[ref]