21666839 has 4 divisors (see below), whose sum is σ = 21810480. Its totient is φ = 21523200.

The previous prime is 21666829. The next prime is 21666863. The reversal of 21666839 is 93866612.

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 21666839 - 2⁸ = 21666583 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21666796 and 21666805.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21666829) 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, 71594 + ... + 71895.

It is an arithmetic number, because the mean of its divisors is an integer number (5452620).

Almost surely, 2^21666839 is an apocalyptic number.

21666839 is a deficient number, since it is larger than the sum of its proper divisors (143641).

21666839 is a wasteful number, since it uses less digits than its factorization.

21666839 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 143640.

The product of its digits is 93312, while the sum is 41.

The square root of 21666839 is about 4654.7651927890. The cubic root of 21666839 is about 278.7822927055.

The spelling of 21666839 in words is "twenty-one million, six hundred sixty-six thousand, eight hundred thirty-nine".