Source: wikibot/perfect-digit-to-digit-invariant
= Perfect digit-to-digit invariant
{wiki=Perfect_digit-to-digit_invariant}
A Perfect Digit-to-Digit Invariant (PDDI) is a property of certain types of number transformations that maintain certain characteristics while altering their form. Specifically, it typically refers to a function or operation that transforms a number or a sequence of digits in such a way that each digit in the input number corresponds directly to a digit in the output through a specific relationship.