21256661 has 2 divisors, whose sum is σ = 21256662. Its totient is φ = 21256660.

The previous prime is 21256637. The next prime is 21256673. The reversal of 21256661 is 16665212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16443025 + 4813636 = 4055^2 + 2194^2 .

It is a cyclic number.

It is not a de Polignac number, because 21256661 - 2⁶ = 21256597 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21256691) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10628330 + 10628331.

It is an arithmetic number, because the mean of its divisors is an integer number (10628331).

Almost surely, 2^21256661 is an apocalyptic number.

It is an amenable number.

21256661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21256661 is an equidigital number, since it uses as much as digits as its factorization.

21256661 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4320, while the sum is 29.

The square root of 21256661 is about 4610.4946589276. The cubic root of 21256661 is about 277.0118507270.

The spelling of 21256661 in words is "twenty-one million, two hundred fifty-six thousand, six hundred sixty-one".