Fredholl numbers are number, which have 2 different digits, equal number of each one, and zero cannot be the first digit. Some examples of fredholl numbers are: 355533 and 4141.
233233, 535351 or 055050 are not fredholl numbers.
In case of 4 digit numbers let's look at the possible forms of Fredholl numbers:
They have to be one of the following: xxyy, xyxy, xyyx
We can write above numbers as:
All 4-digit Fredholl numbers are composite because they will be divisible by either 11 or 101.