20557721 has 2 divisors, whose sum is σ = 20557722. Its totient is φ = 20557720.

The previous prime is 20557711. The next prime is 20557729. The reversal of 20557721 is 12775502.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12355225 + 8202496 = 3515^2 + 2864^2 .

It is a cyclic number.

It is not a de Polignac number, because 20557721 - 2¹⁸ = 20295577 is a prime.

It is not a weakly prime, because it can be changed into another prime (20557729) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10278860 + 10278861.

It is an arithmetic number, because the mean of its divisors is an integer number (10278861).

Almost surely, 2^20557721 is an apocalyptic number.

It is an amenable number.

20557721 is a deficient number, since it is larger than the sum of its proper divisors (1).

20557721 is an equidigital number, since it uses as much as digits as its factorization.

20557721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 4900, while the sum is 29.

The square root of 20557721 is about 4534.0623065856. The cubic root of 20557721 is about 273.9418110401.

The spelling of 20557721 in words is "twenty million, five hundred fifty-seven thousand, seven hundred twenty-one".