21185821 has 2 divisors, whose sum is σ = 21185822. Its totient is φ = 21185820.

The previous prime is 21185789. The next prime is 21185837. The reversal of 21185821 is 12858112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15960025 + 5225796 = 3995^2 + 2286^2 .

It is a cyclic number.

It is not a de Polignac number, because 21185821 - 2⁵ = 21185789 is a prime.

It is a super-2 number, since 2×21185821² = 897678022888082, which contains 22 as substring.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21185821.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21185881) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10592910 + 10592911.

It is an arithmetic number, because the mean of its divisors is an integer number (10592911).

Almost surely, 2^21185821 is an apocalyptic number.

It is an amenable number.

21185821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21185821 is an equidigital number, since it uses as much as digits as its factorization.

21185821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1280, while the sum is 28.

The square root of 21185821 is about 4602.8057747422. The cubic root of 21185821 is about 276.7037847975.

The spelling of 21185821 in words is "twenty-one million, one hundred eighty-five thousand, eight hundred twenty-one".