21491721 has 6 divisors (see below), whose sum is σ = 31043610. Its totient is φ = 14327808.

The previous prime is 21491693. The next prime is 21491741. The reversal of 21491721 is 12719412.

It can be written as a sum of positive squares in only one way, i.e., 21261321 + 230400 = 4611^2 + 480^2 .

It is not a de Polignac number, because 21491721 - 2⁹ = 21491209 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21491741) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1193976 + ... + 1193993.

It is an arithmetic number, because the mean of its divisors is an integer number (5173935).

Almost surely, 2^21491721 is an apocalyptic number.

It is an amenable number.

21491721 is a deficient number, since it is larger than the sum of its proper divisors (9551889).

21491721 is a wasteful number, since it uses less digits than its factorization.

21491721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2387975 (or 2387972 counting only the distinct ones).

The product of its digits is 1008, while the sum is 27.

The square root of 21491721 is about 4635.9164142594. The cubic root of 21491721 is about 278.0291922832.

The spelling of 21491721 in words is "twenty-one million, four hundred ninety-one thousand, seven hundred twenty-one".