21512391 has 4 divisors (see below), whose sum is σ = 28683192. Its totient is φ = 14341592.

The previous prime is 21512371. The next prime is 21512401. The reversal of 21512391 is 19321512.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21512391 - 2⁷ = 21512263 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21512311) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3585396 + ... + 3585401.

It is an arithmetic number, because the mean of its divisors is an integer number (7170798).

Almost surely, 2^21512391 is an apocalyptic number.

21512391 is a deficient number, since it is larger than the sum of its proper divisors (7170801).

21512391 is an equidigital number, since it uses as much as digits as its factorization.

21512391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7170800.

The product of its digits is 540, while the sum is 24.

The square root of 21512391 is about 4638.1452111809. The cubic root of 21512391 is about 278.1182967033.

The spelling of 21512391 in words is "twenty-one million, five hundred twelve thousand, three hundred ninety-one".