21871721 has 2 divisors, whose sum is σ = 21871722. Its totient is φ = 21871720.

The previous prime is 21871667. The next prime is 21871723. The reversal of 21871721 is 12717812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15054400 + 6817321 = 3880^2 + 2611^2 .

It is a cyclic number.

It is not a de Polignac number, because 21871721 - 2²² = 17677417 is a prime.

It is a Sophie Germain prime.

Together with 21871723, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21871723) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10935860 + 10935861.

It is an arithmetic number, because the mean of its divisors is an integer number (10935861).

Almost surely, 2^21871721 is an apocalyptic number.

It is an amenable number.

21871721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21871721 is an equidigital number, since it uses as much as digits as its factorization.

21871721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1568, while the sum is 29.

The square root of 21871721 is about 4676.7211804853. The cubic root of 21871721 is about 279.6582607991.

The spelling of 21871721 in words is "twenty-one million, eight hundred seventy-one thousand, seven hundred twenty-one".