21005813 has 2 divisors, whose sum is σ = 21005814. Its totient is φ = 21005812.

The previous prime is 21005807. The next prime is 21005821. The reversal of 21005813 is 31850012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15816529 + 5189284 = 3977^2 + 2278^2 .

It is a cyclic number.

It is not a de Polignac number, because 21005813 - 2⁴ = 21005797 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21005713) 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, 10502906 + 10502907.

It is an arithmetic number, because the mean of its divisors is an integer number (10502907).

Almost surely, 2^21005813 is an apocalyptic number.

It is an amenable number.

21005813 is a deficient number, since it is larger than the sum of its proper divisors (1).

21005813 is an equidigital number, since it uses as much as digits as its factorization.

21005813 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 240, while the sum is 20.

The square root of 21005813 is about 4583.2099013683. The cubic root of 21005813 is about 275.9178718392.

Adding to 21005813 its reverse (31850012), we get a palindrome (52855825).

The spelling of 21005813 in words is "twenty-one million, five thousand, eight hundred thirteen".