 Harshad number

A Harshad number in base $b\ge 2$ is a positive integer divisible by its base-p digit sum in that base. In base ten, $18$ is a Harshad number because $1+8=9$ divides $18$. It is strong when the quotient by its base-p digit sum is prime, and right-truncatable when every nonempty prefix is a Harshad number.