21666021 has 16 divisors (see below), whose sum is σ = 31206336. Its totient is φ = 13291776.

The previous prime is 21666019. The next prime is 21666037. The reversal of 21666021 is 12066612.

It is not a de Polignac number, because 21666021 - 2¹ = 21666019 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21665982 and 21666000.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21666091) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 16246 + ... + 17528.

It is an arithmetic number, because the mean of its divisors is an integer number (1950396).

Almost surely, 2^21666021 is an apocalyptic number.

It is an amenable number.

21666021 is a deficient number, since it is larger than the sum of its proper divisors (9540315).

21666021 is a wasteful number, since it uses less digits than its factorization.

21666021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1732.

The product of its (nonzero) digits is 864, while the sum is 24.

The square root of 21666021 is about 4654.6773250141. The cubic root of 21666021 is about 278.7787843214.

The spelling of 21666021 in words is "twenty-one million, six hundred sixty-six thousand, twenty-one".