Mathematical Girls - Vol. 1 Ch. 2

[MangaDex]

 

[Nylodale]

For further information on why 1 is not a prime number, you can look up Numberphile's YouTube video on it, titled "1 and Prime Numbers - Numberphile" https://youtu.be/IQofiPqhJ_s.

 

[Alraisen]

Well, there can only be one number one prime factor for an integer, after all.

 

[Rusian]

For a moment, I was worried about the rigidness of translation, but it seems I have nothing to worry about.