21218399 has 4 divisors (see below), whose sum is σ = 21509136. Its totient is φ = 20927664.

The previous prime is 21218387. The next prime is 21218413. The reversal of 21218399 is 99381212.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21218399 - 2⁸ = 21218143 is a prime.

It is a super-2 number, since 2×21218399² = 900440912246402, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21218359) 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 in 3 ways as a sum of consecutive naturals, for example, 145259 + ... + 145404.

It is an arithmetic number, because the mean of its divisors is an integer number (5377284).

Almost surely, 2^21218399 is an apocalyptic number.

21218399 is a deficient number, since it is larger than the sum of its proper divisors (290737).

21218399 is an equidigital number, since it uses as much as digits as its factorization.

21218399 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 290736.

The product of its digits is 7776, while the sum is 35.

The square root of 21218399 is about 4606.3433436946. The cubic root of 21218399 is about 276.8455437481.

The spelling of 21218399 in words is "twenty-one million, two hundred eighteen thousand, three hundred ninety-nine".