20921609 has 4 divisors (see below), whose sum is σ = 21037380. Its totient is φ = 20805840.

The previous prime is 20921519. The next prime is 20921623. The reversal of 20921609 is 90612902.

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 7997584 + 12924025 = 2828^2 + 3595^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20921609 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20921629) 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, 57614 + ... + 57975.

It is an arithmetic number, because the mean of its divisors is an integer number (5259345).

Almost surely, 2^20921609 is an apocalyptic number.

It is an amenable number.

20921609 is a deficient number, since it is larger than the sum of its proper divisors (115771).

20921609 is a wasteful number, since it uses less digits than its factorization.

20921609 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 115770.

The product of its (nonzero) digits is 1944, while the sum is 29.

The square root of 20921609 is about 4574.0145386739. The cubic root of 20921609 is about 275.5486962164.

The spelling of 20921609 in words is "twenty million, nine hundred twenty-one thousand, six hundred nine".