20720921 has 6 divisors (see below), whose sum is σ = 22437630. Its totient is φ = 19126848.

The previous prime is 20720911. The next prime is 20720951. The reversal of 20720921 is 12902702.

It can be written as a sum of positive squares in 3 ways, for example, as 290521 + 20430400 = 539^2 + 4520^2 .

It is not a de Polignac number, because 20720921 - 2¹⁸ = 20458777 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20720911) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 61136 + ... + 61473.

It is an arithmetic number, because the mean of its divisors is an integer number (3739605).

Almost surely, 2^20720921 is an apocalyptic number.

It is an amenable number.

20720921 is a deficient number, since it is larger than the sum of its proper divisors (1716709).

20720921 is a wasteful number, since it uses less digits than its factorization.

20720921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 122635 (or 122622 counting only the distinct ones).

The product of its (nonzero) digits is 504, while the sum is 23.

The square root of 20720921 is about 4552.0238356142. The cubic root of 20720921 is about 274.6648081364.

The spelling of 20720921 in words is "twenty million, seven hundred twenty thousand, nine hundred twenty-one".