21615391 has 8 divisors (see below), whose sum is σ = 24935040. Its totient is φ = 18353688.

The previous prime is 21615379. The next prime is 21615409. The reversal of 21615391 is 19351612.

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

It is a cyclic number.

It is not a de Polignac number, because 21615391 - 2⁵ = 21615359 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21615301) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 13681 + ... + 15178.

It is an arithmetic number, because the mean of its divisors is an integer number (3116880).

Almost surely, 2^21615391 is an apocalyptic number.

21615391 is a deficient number, since it is larger than the sum of its proper divisors (3319649).

21615391 is a wasteful number, since it uses less digits than its factorization.

21615391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 28973.

The product of its digits is 1620, while the sum is 28.

The square root of 21615391 is about 4649.2355285574. The cubic root of 21615391 is about 278.5614612507.

The spelling of 21615391 in words is "twenty-one million, six hundred fifteen thousand, three hundred ninety-one".