21317287 has 4 divisors (see below), whose sum is σ = 21328200. Its totient is φ = 21306376.

The previous prime is 21317267. The next prime is 21317327. The reversal of 21317287 is 78271312.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21317287 - 2¹⁵ = 21284519 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21317267) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1633 + ... + 6730.

It is an arithmetic number, because the mean of its divisors is an integer number (5332050).

Almost surely, 2^21317287 is an apocalyptic number.

21317287 is a deficient number, since it is larger than the sum of its proper divisors (10913).

21317287 is an equidigital number, since it uses as much as digits as its factorization.

21317287 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10912.

The product of its digits is 4704, while the sum is 31.

The square root of 21317287 is about 4617.0647602129. The cubic root of 21317287 is about 277.2749553851.

Adding to 21317287 its reverse (78271312), we get a palindrome (99588599).

The spelling of 21317287 in words is "twenty-one million, three hundred seventeen thousand, two hundred eighty-seven".