21019809 has 16 divisors (see below), whose sum is σ = 29116800. Its totient is φ = 13472256.

The previous prime is 21019799. The next prime is 21019841. The reversal of 21019809 is 90891012.

It is not a de Polignac number, because 21019809 - 2⁴ = 21019793 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21019849) 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, 30897 + ... + 31569.

It is an arithmetic number, because the mean of its divisors is an integer number (1819800).

Almost surely, 2^21019809 is an apocalyptic number.

21019809 is a gapful number since it is divisible by the number (29) formed by its first and last digit.

It is an amenable number.

21019809 is a deficient number, since it is larger than the sum of its proper divisors (8096991).

21019809 is a wasteful number, since it uses less digits than its factorization.

21019809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1064.

The product of its (nonzero) digits is 1296, while the sum is 30.

The square root of 21019809 is about 4584.7365245999. The cubic root of 21019809 is about 275.9791388349.

The spelling of 21019809 in words is "twenty-one million, nineteen thousand, eight hundred nine".