21287 has 4 divisors (see below), whose sum is σ = 24336. Its totient is φ = 18240.

The previous prime is 21283. The next prime is 21313. The reversal of 21287 is 78212.

21287 = T₄₈ + T₄₉ + ... + T₆₁.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21287 - 2² = 21283 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21287.

It is not an unprimeable number, because it can be changed into a prime (21283) 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, 1514 + ... + 1527.

It is an arithmetic number, because the mean of its divisors is an integer number (6084).

2²¹²⁸⁷ is an apocalyptic number.

21287 is a deficient number, since it is larger than the sum of its proper divisors (3049).

21287 is an equidigital number, since it uses as much as digits as its factorization.

21287 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3048.

The product of its digits is 224, while the sum is 20.

The square root of 21287 is about 145.9006511295. The cubic root of 21287 is about 27.7143578335.

Adding to 21287 its reverse (78212), we get a palindrome (99499).

The spelling of 21287 in words is "twenty-one thousand, two hundred eighty-seven".