21761721 has 12 divisors (see below), whose sum is σ = 32448000. Its totient is φ = 14039640.

The previous prime is 21761711. The next prime is 21761723. The reversal of 21761721 is 12716712.

21761721 is a `hidden beast` number, since 21 + 7 + 617 + 21 = 666.

It is not a de Polignac number, because 21761721 - 2¹⁴ = 21745337 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21761723) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 38721 + ... + 39278.

It is an arithmetic number, because the mean of its divisors is an integer number (2704000).

Almost surely, 2^21761721 is an apocalyptic number.

It is an amenable number.

21761721 is a deficient number, since it is larger than the sum of its proper divisors (10686279).

21761721 is a wasteful number, since it uses less digits than its factorization.

21761721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 78036 (or 78033 counting only the distinct ones).

The product of its digits is 1176, while the sum is 27.

The square root of 21761721 is about 4664.9459803946. The cubic root of 21761721 is about 279.1886418384.

The spelling of 21761721 in words is "twenty-one million, seven hundred sixty-one thousand, seven hundred twenty-one".