20728921 has 2 divisors, whose sum is σ = 20728922. Its totient is φ = 20728920.

The previous prime is 20728919. The next prime is 20728949. The reversal of 20728921 is 12982702.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15840400 + 4888521 = 3980^2 + 2211^2 .

It is a cyclic number.

It is not a de Polignac number, because 20728921 - 2¹ = 20728919 is a prime.

Together with 20728919, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (20728121) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10364460 + 10364461.

It is an arithmetic number, because the mean of its divisors is an integer number (10364461).

Almost surely, 2^20728921 is an apocalyptic number.

It is an amenable number.

20728921 is a deficient number, since it is larger than the sum of its proper divisors (1).

20728921 is an equidigital number, since it uses as much as digits as its factorization.

20728921 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4032, while the sum is 31.

The square root of 20728921 is about 4552.9024808357. The cubic root of 20728921 is about 274.7001514133.

The spelling of 20728921 in words is "twenty million, seven hundred twenty-eight thousand, nine hundred twenty-one".