21991515 has 32 divisors (see below), whose sum is σ = 43309056. Its totient is φ = 9279360.

The previous prime is 21991499. The next prime is 21991517. The reversal of 21991515 is 51519912.

It is a happy number.

It is not a de Polignac number, because 21991515 - 2⁴ = 21991499 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21991517) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 6691 + ... + 9420.

It is an arithmetic number, because the mean of its divisors is an integer number (1353408).

Almost surely, 2^21991515 is an apocalyptic number.

21991515 is a deficient number, since it is larger than the sum of its proper divisors (21317541).

21991515 is a wasteful number, since it uses less digits than its factorization.

21991515 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 16139.

The product of its digits is 4050, while the sum is 33.

The square root of 21991515 is about 4689.5111685548. The cubic root of 21991515 is about 280.1679052459.

The spelling of 21991515 in words is "twenty-one million, nine hundred ninety-one thousand, five hundred fifteen".