The crisp new dollar bill that I have just taken from my wallet bears the serial number 24598176. It’s easy to tell that the number is exactly divisible by 2 but not by 5. Is it divisible by 3? by 4?

Prime numbers, vital in mathematics and competitive exams, can be quickly identified using a systematic approach. This method involves checking divisibility by primes up to the square root of the ...

Three and five are prime numbers — that is, they are divisible only by one and by themselves. So are five and seven. And 11 and 13. And for each of these pairs of prime numbers, the difference is two.

One of my favorite anecdotes about prime numbers concerns Alexander Grothendieck, who was among the most brilliant mathematicians of the 20th century. According to one account, he was once asked to ...