1 is a...
We are lucky we won WWII. Our training of soldiers was in error.
Consider the resource Mathematics: Volume 1 of the Navy Training Course Manual, (NAVPERS 10069-C).
As defined on page 17, "A number that has no factors except itself and 1 is a PRIME NUMBER."
The definition is followed by this claim: "The following series shows all the prime numbers up to 60: 1, 2, 3, 5, 7, 11, 13, 17....."
That's right...according to the Navy in the 1940s, 1 was a prime number. And, we won the war!
Yet, I wonder, have the Navy's manuals been changed now?