Shorter factor strings aren't composed entirely of prime numbers.
It varies with each given number.
It is a square of that factor. E.g. 5 is a distinct factor of 25. If you multiply that distinct factor by itself (5*5) you get 25. A square comes from the x^2 notation and is any number which has a factor which is multiplied by itself.
The difference is between factor pairs and distinct factors. With square numbers, one of the factor pairs will be the same number twice. When listing the distinct factors, that number is only listed once.
5 x 5
The longest factor string of any composite number is its prime factorization.
The longest factor string is 2x2x2x3x13. It is always best to list your factor string from the least to greatest number.
For composite numbers, only one string of factors is the longest; the prime factorization.
A perfect square
No, 12 is not a prime number.
10 of them.
The prime factorization