20509921 has 2 divisors, whose sum is σ = 20509922. Its totient is φ = 20509920.

The previous prime is 20509919. The next prime is 20509933. The reversal of 20509921 is 12990502.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19580625 + 929296 = 4425^2 + 964^2 .

It is a cyclic number.

It is not a de Polignac number, because 20509921 - 2¹ = 20509919 is a prime.

Together with 20509919, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 20509921.

It is not a weakly prime, because it can be changed into another prime (20509981) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10254960 + 10254961.

It is an arithmetic number, because the mean of its divisors is an integer number (10254961).

Almost surely, 2^20509921 is an apocalyptic number.

It is an amenable number.

20509921 is a deficient number, since it is larger than the sum of its proper divisors (1).

20509921 is an equidigital number, since it uses as much as digits as its factorization.

20509921 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1620, while the sum is 28.

The square root of 20509921 is about 4528.7880277178. The cubic root of 20509921 is about 273.7293267125.

It can be divided in two parts, 205099 and 21, that added together give a triangular number (205120 = T₆₄₀).

The spelling of 20509921 in words is "twenty million, five hundred nine thousand, nine hundred twenty-one".