21251309 has 8 divisors (see below), whose sum is σ = 22726008. Its totient is φ = 19801600.

The previous prime is 21251299. The next prime is 21251311. The reversal of 21251309 is 90315212.

It can be written as a sum of positive squares in 4 ways, for example, as 7038409 + 14212900 = 2653^2 + 3770^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21251309 - 2⁸ = 21251053 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21251339) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4472 + ... + 7905.

It is an arithmetic number, because the mean of its divisors is an integer number (2840751).

Almost surely, 2^21251309 is an apocalyptic number.

It is an amenable number.

21251309 is a deficient number, since it is larger than the sum of its proper divisors (1474699).

21251309 is a wasteful number, since it uses less digits than its factorization.

21251309 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12495.

The product of its (nonzero) digits is 540, while the sum is 23.

The square root of 21251309 is about 4609.9142074446. The cubic root of 21251309 is about 276.9886001034.

The spelling of 21251309 in words is "twenty-one million, two hundred fifty-one thousand, three hundred nine".