21020509 has 2 divisors, whose sum is σ = 21020510. Its totient is φ = 21020508.

The previous prime is 21020501. The next prime is 21020521. The reversal of 21020509 is 90502012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20748025 + 272484 = 4555^2 + 522^2 .

It is a cyclic number.

It is not a de Polignac number, because 21020509 - 2³ = 21020501 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21020501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10510254 + 10510255.

It is an arithmetic number, because the mean of its divisors is an integer number (10510255).

Almost surely, 2^21020509 is an apocalyptic number.

It is an amenable number.

21020509 is a deficient number, since it is larger than the sum of its proper divisors (1).

21020509 is an equidigital number, since it uses as much as digits as its factorization.

21020509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 180, while the sum is 19.

The square root of 21020509 is about 4584.8128642290. The cubic root of 21020509 is about 275.9822023460.

The spelling of 21020509 in words is "twenty-one million, twenty thousand, five hundred nine".