21012309 has 24 divisors (see below), whose sum is σ = 32285760. Its totient is φ = 13131072.

The previous prime is 21012307. The next prime is 21012311. The reversal of 21012309 is 90321012.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21012307) and next prime (21012311).

It is not a de Polignac number, because 21012309 - 2¹ = 21012307 is a prime.

It is a super-2 number, since 2×21012309² = 883034259022962, which contains 22 as substring.

It is an alternating number because its digits alternate between even and odd.

It is a junction number, because it is equal to n+sod(n) for n = 21012291 and 21012300.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21012301) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 17017 + ... + 18209.

It is an arithmetic number, because the mean of its divisors is an integer number (1345240).

Almost surely, 2^21012309 is an apocalyptic number.

It is an amenable number.

21012309 is a deficient number, since it is larger than the sum of its proper divisors (11273451).

21012309 is a wasteful number, since it uses less digits than its factorization.

21012309 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1321 (or 1318 counting only the distinct ones).

The product of its (nonzero) digits is 108, while the sum is 18.

The square root of 21012309 is about 4583.9185202183. The cubic root of 21012309 is about 275.9463112330.

It can be divided in two parts, 21012 and 309, that added together give a triangular number (21321 = T₂₀₆).

The spelling of 21012309 in words is "twenty-one million, twelve thousand, three hundred nine".