21721917 has 32 divisors (see below), whose sum is σ = 34791424. Its totient is φ = 11793600.

The previous prime is 21721907. The next prime is 21721927. The reversal of 21721917 is 71912712.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21721907) and next prime (21721927).

It is not a de Polignac number, because 21721917 - 2⁴ = 21721901 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21721907) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 39438 + ... + 39984.

It is an arithmetic number, because the mean of its divisors is an integer number (1087232).

Almost surely, 2^21721917 is an apocalyptic number.

It is an amenable number.

21721917 is a deficient number, since it is larger than the sum of its proper divisors (13069507).

21721917 is a wasteful number, since it uses less digits than its factorization.

21721917 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 649.

The product of its digits is 1764, while the sum is 30.

The square root of 21721917 is about 4660.6777404150. The cubic root of 21721917 is about 279.0183181919.

The spelling of 21721917 in words is "twenty-one million, seven hundred twenty-one thousand, nine hundred seventeen".