21640721 has 2 divisors, whose sum is σ = 21640722. Its totient is φ = 21640720.

The previous prime is 21640693. The next prime is 21640747. The reversal of 21640721 is 12704612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21492496 + 148225 = 4636^2 + 385^2 .

It is a cyclic number.

It is not a de Polignac number, because 21640721 - 2²² = 17446417 is a prime.

It is not a weakly prime, because it can be changed into another prime (21640121) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10820360 + 10820361.

It is an arithmetic number, because the mean of its divisors is an integer number (10820361).

Almost surely, 2^21640721 is an apocalyptic number.

It is an amenable number.

21640721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21640721 is an equidigital number, since it uses as much as digits as its factorization.

21640721 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 672, while the sum is 23.

The square root of 21640721 is about 4651.9588347276. The cubic root of 21640721 is about 278.6702295419.

The spelling of 21640721 in words is "twenty-one million, six hundred forty thousand, seven hundred twenty-one".