21665641 has 2 divisors, whose sum is σ = 21665642. Its totient is φ = 21665640.

The previous prime is 21665587. The next prime is 21665647. The reversal of 21665641 is 14656612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19105641 + 2560000 = 4371^2 + 1600^2 .

It is a cyclic number.

It is not a de Polignac number, because 21665641 - 2⁹ = 21665129 is a prime.

It is not a weakly prime, because it can be changed into another prime (21665647) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10832820 + 10832821.

It is an arithmetic number, because the mean of its divisors is an integer number (10832821).

Almost surely, 2^21665641 is an apocalyptic number.

It is an amenable number.

21665641 is a deficient number, since it is larger than the sum of its proper divisors (1).

21665641 is an equidigital number, since it uses as much as digits as its factorization.

21665641 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 8640, while the sum is 31.

The square root of 21665641 is about 4654.6365056790. The cubic root of 21665641 is about 278.7771544796. Note that the first 3 decimals are identical.

The spelling of 21665641 in words is "twenty-one million, six hundred sixty-five thousand, six hundred forty-one".