Primorial
Let \(p_n\) be the \(n\)th prime, so \(p_1=2,p_2=3,p_3=5,p_4=7,\ldots\). The \(n\)th primorial number is \(p_n\#=p_1p_2\ldots p_n\).
These numbers are products of many small numbers and easy to factor, but adding or subtracting 1 makes most hard to factor.