21239987 has 4 divisors (see below), whose sum is σ = 22489416. Its totient is φ = 19990560.

The previous prime is 21239969. The next prime is 21240001. The reversal of 21239987 is 78993212.

It is a happy number.

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 21239987 - 2⁶ = 21239923 is a prime.

It is a super-2 number, since 2×21239987² = 902274095520338, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21239917) 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, 624689 + ... + 624722.

It is an arithmetic number, because the mean of its divisors is an integer number (5622354).

Almost surely, 2^21239987 is an apocalyptic number.

21239987 is a deficient number, since it is larger than the sum of its proper divisors (1249429).

21239987 is a wasteful number, since it uses less digits than its factorization.

21239987 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1249428.

The product of its digits is 54432, while the sum is 41.

The square root of 21239987 is about 4608.6860383411. The cubic root of 21239987 is about 276.9394012199.

The spelling of 21239987 in words is "twenty-one million, two hundred thirty-nine thousand, nine hundred eighty-seven".