1 is a sort of an oddball.

A prime number has exactly two integer divisors, 1 and itself.

A composite number has 3 or more divisors.

1 only has a singular divisor, itself. It is the only counting number with this property.

For most mathematical purposes, 1 isn't considered 'prime', but rather a multiplicative identity. For instance, 1 isn't listed for prime factorization of a number.

That being said, it would seem for the purposes of this pattern, 1 is considered prime, and as such I will accept it.