21706921 has 2 divisors, whose sum is σ = 21706922. Its totient is φ = 21706920.

The previous prime is 21706897. The next prime is 21706931. The reversal of 21706921 is 12960712.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19456921 + 2250000 = 4411^2 + 1500^2 .

It is a cyclic number.

It is not a de Polignac number, because 21706921 - 2⁵ = 21706889 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21706921.

It is not a weakly prime, because it can be changed into another prime (21706931) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10853460 + 10853461.

It is an arithmetic number, because the mean of its divisors is an integer number (10853461).

Almost surely, 2^21706921 is an apocalyptic number.

It is an amenable number.

21706921 is a deficient number, since it is larger than the sum of its proper divisors (1).

21706921 is an equidigital number, since it uses as much as digits as its factorization.

21706921 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1512, while the sum is 28.

The square root of 21706921 is about 4659.0686837607. The cubic root of 21706921 is about 278.9540954696.

The spelling of 21706921 in words is "twenty-one million, seven hundred six thousand, nine hundred twenty-one".