21811813 has 2 divisors, whose sum is σ = 21811814. Its totient is φ = 21811812.

The previous prime is 21811771. The next prime is 21811837. The reversal of 21811813 is 31811812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20412324 + 1399489 = 4518^2 + 1183^2 .

It is a cyclic number.

It is not a de Polignac number, because 21811813 - 2¹⁷ = 21680741 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21811873) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10905906 + 10905907.

It is an arithmetic number, because the mean of its divisors is an integer number (10905907).

Almost surely, 2^21811813 is an apocalyptic number.

It is an amenable number.

21811813 is a deficient number, since it is larger than the sum of its proper divisors (1).

21811813 is an equidigital number, since it uses as much as digits as its factorization.

21811813 is an evil number, because the sum of its binary digits is even.

The product of its digits is 384, while the sum is 25.

The square root of 21811813 is about 4670.3118739545. The cubic root of 21811813 is about 279.4026935464.

Subtracting 21811813 from its reverse (31811812), we obtain a palindrome (9999999).

The spelling of 21811813 in words is "twenty-one million, eight hundred eleven thousand, eight hundred thirteen".