639 is not a prime number.

A prime number has only two positive divisors: 1 and itself.

For 639, the positive divisors are :

• 1 is a divisor of 639 : $639 / 1 = 639$
• 3 is a divisor of 639 : $639 / 3 = 213$
• 9 is a divisor of 639 : $639 / 9 = 71$
• 71 is a divisor of 639 : $639 / 71 = 9$
• 213 is a divisor of 639 : $639 / 213 = 3$
• 639 is a divisor of 639 : $639 / 639 = 1$

Since 639 is divisible by 3, 9, 71 and 213, it has divisors other than 1 and itself.

Therefore, 639 is not a prime number.