21006921 has 16 divisors (see below), whose sum is σ = 30735936. Its totient is φ = 12682176.

The previous prime is 21006907. The next prime is 21006941. The reversal of 21006921 is 12960012.

It is not a de Polignac number, because 21006921 - 2⁷ = 21006793 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21006941) 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, 3015 + ... + 7148.

It is an arithmetic number, because the mean of its divisors is an integer number (1920996).

Almost surely, 2^21006921 is an apocalyptic number.

It is an amenable number.

21006921 is a deficient number, since it is larger than the sum of its proper divisors (9729015).

21006921 is a wasteful number, since it uses less digits than its factorization.

21006921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10232.

The product of its (nonzero) digits is 216, while the sum is 21.

The square root of 21006921 is about 4583.3307757569. The cubic root of 21006921 is about 275.9227230618.

Adding to 21006921 its reverse (12960012), we get a palindrome (33966933).

The spelling of 21006921 in words is "twenty-one million, six thousand, nine hundred twenty-one".