21408021 has 6 divisors (see below), whose sum is σ = 30922710. Its totient is φ = 14272008.

The previous prime is 21408019. The next prime is 21408029. The reversal of 21408021 is 12080412.

It can be written as a sum of positive squares in only one way, i.e., 21288996 + 119025 = 4614^2 + 345^2 .

It is not a de Polignac number, because 21408021 - 2¹ = 21408019 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21407985 and 21408003.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21408029) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1189326 + ... + 1189343.

It is an arithmetic number, because the mean of its divisors is an integer number (5153785).

Almost surely, 2^21408021 is an apocalyptic number.

It is an amenable number.

21408021 is a deficient number, since it is larger than the sum of its proper divisors (9514689).

21408021 is a wasteful number, since it uses less digits than its factorization.

21408021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2378675 (or 2378672 counting only the distinct ones).

The product of its (nonzero) digits is 128, while the sum is 18.

The square root of 21408021 is about 4626.8802664430. The cubic root of 21408021 is about 277.6677923625.

Adding to 21408021 its reverse (12080412), we get a palindrome (33488433).

The spelling of 21408021 in words is "twenty-one million, four hundred eight thousand, twenty-one".