21865721 has 2 divisors, whose sum is σ = 21865722. Its totient is φ = 21865720.

The previous prime is 21865687. The next prime is 21865741. The reversal of 21865721 is 12756812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15912121 + 5953600 = 3989^2 + 2440^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21865721 is a prime.

It is not a weakly prime, because it can be changed into another prime (21865741) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10932860 + 10932861.

It is an arithmetic number, because the mean of its divisors is an integer number (10932861).

Almost surely, 2^21865721 is an apocalyptic number.

It is an amenable number.

21865721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21865721 is an equidigital number, since it uses as much as digits as its factorization.

21865721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 6720, while the sum is 32.

The square root of 21865721 is about 4676.0796614258. The cubic root of 21865721 is about 279.6326858718.

The spelling of 21865721 in words is "twenty-one million, eight hundred sixty-five thousand, seven hundred twenty-one".