21016909 has 4 divisors (see below), whose sum is σ = 21741660. Its totient is φ = 20292160.

The previous prime is 21016903. The next prime is 21016949. The reversal of 21016909 is 90961012.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 5978025 + 15038884 = 2445^2 + 3878^2 .

It is a cyclic number.

It is not a de Polignac number, because 21016909 - 2³ = 21016901 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21016901) 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, 362332 + ... + 362389.

It is an arithmetic number, because the mean of its divisors is an integer number (5435415).

Almost surely, 2^21016909 is an apocalyptic number.

21016909 is a gapful number since it is divisible by the number (29) formed by its first and last digit.

It is an amenable number.

21016909 is a deficient number, since it is larger than the sum of its proper divisors (724751).

21016909 is an equidigital number, since it uses as much as digits as its factorization.

21016909 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 724750.

The product of its (nonzero) digits is 972, while the sum is 28.

The square root of 21016909 is about 4584.4202468796. The cubic root of 21016909 is about 275.9664464216.

The spelling of 21016909 in words is "twenty-one million, sixteen thousand, nine hundred nine".