21021409 has 4 divisors (see below), whose sum is σ = 21030804. Its totient is φ = 21012016.

The previous prime is 21021389. The next prime is 21021419. The reversal of 21021409 is 90412012.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 14630625 + 6390784 = 3825^2 + 2528^2 .

It is a cyclic number.

It is not a de Polignac number, because 21021409 - 2¹³ = 21013217 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21021419) 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, 819 + ... + 6535.

It is an arithmetic number, because the mean of its divisors is an integer number (5257701).

Almost surely, 2^21021409 is an apocalyptic number.

It is an amenable number.

21021409 is a deficient number, since it is larger than the sum of its proper divisors (9395).

21021409 is an equidigital number, since it uses as much as digits as its factorization.

21021409 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9394.

The product of its (nonzero) digits is 144, while the sum is 19.

The square root of 21021409 is about 4584.9110133131. The cubic root of 21021409 is about 275.9861410460.

The spelling of 21021409 in words is "twenty-one million, twenty-one thousand, four hundred nine".