Complex sentence: In number theory, a narcissistic number or pluperfect digital invariant (PPDI) or Armstrong number is a a number that is the sum of its own digits to the power of the number of digits.
Split into simpler sentences: In number theory, a narcissistic number is a number that is representable by mathematical manipulation of the digits of the number itself. A Pluperfect Digital Invariant (PPDI) or Armstrong number is a number that is the sum of its own digits to the power of the number of digits.