21256765209 has 27 divisors (see below), whose sum is σ = 32112421887. Its totient is φ = 13548623616.

The previous prime is 21256765181. The next prime is 21256765217. The reversal of 21256765209 is 90256765212.

The square root of 21256765209 is 145797.

It is a perfect power (a square), and thus also a powerful number.

21256765209 is a `hidden beast` number, since 2 + 1 + 2 + 56 + 76 + 520 + 9 = 666.

It can be written as a sum of positive squares in only one way, i.e., 20115225 + 21236649984 = 4485^2 + 145728^2 .

It is not a de Polignac number, because 21256765209 - 2⁵ = 21256765177 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21256765219) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 26 ways as a sum of consecutive naturals, for example, 10058937 + ... + 10061049.

Almost surely, 2^21256765209 is an apocalyptic number.

21256765209 is the 145797-th square number.

21256765209 is the 72899-th centered octagonal number.

It is an amenable number.

21256765209 is a deficient number, since it is larger than the sum of its proper divisors (10855656678).

21256765209 is an frugal number, since it uses more digits than its factorization.

21256765209 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4278 (or 2139 counting only the distinct ones).

The product of its (nonzero) digits is 453600, while the sum is 45.

The spelling of 21256765209 in words is "twenty-one billion, two hundred fifty-six million, seven hundred sixty-five thousand, two hundred nine".