21715991 has 6 divisors (see below), whose sum is σ = 23869776. Its totient is φ = 19741700.

The previous prime is 21715951. The next prime is 21715997. The reversal of 21715991 is 19951712.

It is not a de Polignac number, because 21715991 - 2¹⁴ = 21699607 is a prime.

It is a super-2 number, since 2×21715991² = 943168530224162, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21715997) 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 5 ways as a sum of consecutive naturals, for example, 89615 + ... + 89856.

It is an arithmetic number, because the mean of its divisors is an integer number (3978296).

Almost surely, 2^21715991 is an apocalyptic number.

21715991 is a deficient number, since it is larger than the sum of its proper divisors (2153785).

21715991 is a wasteful number, since it uses less digits than its factorization.

21715991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 179493 (or 179482 counting only the distinct ones).

The product of its digits is 5670, while the sum is 35.

The square root of 21715991 is about 4660.0419526009. The cubic root of 21715991 is about 278.9929427008.

The spelling of 21715991 in words is "twenty-one million, seven hundred fifteen thousand, nine hundred ninety-one".