21123709 has 2 divisors, whose sum is σ = 21123710. Its totient is φ = 21123708.

The previous prime is 21123701. The next prime is 21123721. The reversal of 21123709 is 90732112.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20052484 + 1071225 = 4478^2 + 1035^2 .

It is a cyclic number.

It is not a de Polignac number, because 21123709 - 2³ = 21123701 is a prime.

It is a super-2 number, since 2×21123709² = 892422163833362, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21123701) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10561854 + 10561855.

It is an arithmetic number, because the mean of its divisors is an integer number (10561855).

Almost surely, 2^21123709 is an apocalyptic number.

It is an amenable number.

21123709 is a deficient number, since it is larger than the sum of its proper divisors (1).

21123709 is an equidigital number, since it uses as much as digits as its factorization.

21123709 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 756, while the sum is 25.

The square root of 21123709 is about 4596.0536332815. The cubic root of 21123709 is about 276.4331092885.

The spelling of 21123709 in words is "twenty-one million, one hundred twenty-three thousand, seven hundred nine".