Complex sentence: A number which is a Harshad number in any number base is called an all-Harshad number, or an all-Niven number; there are only four all-Harshad numbers, 1,2,4 and 6.
Split into simpler sentences: A number which is a Harshad number in any number base is called an all-Harshad number, or an all-Niven number. There are only four all-Harshad numbers: 1,2,4, and 6.