21005421 has 16 divisors (see below), whose sum is σ = 30077568. Its totient is φ = 12992000.

The previous prime is 21005401. The next prime is 21005423. The reversal of 21005421 is 12450012.

It is a cyclic number.

It is not a de Polignac number, because 21005421 - 2⁷ = 21005293 is a prime.

It is a super-2 number, since 2×21005421² = 882455422774482, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21005421.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005423) 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, 721 + ... + 6521.

It is an arithmetic number, because the mean of its divisors is an integer number (1879848).

Almost surely, 2^21005421 is an apocalyptic number.

It is an amenable number.

21005421 is a deficient number, since it is larger than the sum of its proper divisors (9072147).

21005421 is a wasteful number, since it uses less digits than its factorization.

21005421 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5892.

The product of its (nonzero) digits is 80, while the sum is 15.

The square root of 21005421 is about 4583.1671363807. The cubic root of 21005421 is about 275.9161554813.

Adding to 21005421 its reverse (12450012), we get a palindrome (33455433).

The spelling of 21005421 in words is "twenty-one million, five thousand, four hundred twenty-one".