21778213 has 4 divisors (see below), whose sum is σ = 22480768. Its totient is φ = 21075660.

The previous prime is 21778201. The next prime is 21778219. The reversal of 21778213 is 31287712.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21778213 - 2⁵ = 21778181 is a prime.

It is a super-2 number, since 2×21778213² = 948581122946738, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (31), and also a Moran number because the ratio is a prime number: 702523 = 21778213 / (2 + 1 + 7 + 7 + 8 + 2 + 1 + 3).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21778219) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 351231 + ... + 351292.

It is an arithmetic number, because the mean of its divisors is an integer number (5620192).

Almost surely, 2^21778213 is an apocalyptic number.

It is an amenable number.

21778213 is a deficient number, since it is larger than the sum of its proper divisors (702555).

21778213 is an equidigital number, since it uses as much as digits as its factorization.

21778213 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 702554.

The product of its digits is 4704, while the sum is 31.

The square root of 21778213 is about 4666.7132973861. The cubic root of 21778213 is about 279.2591512194.

The spelling of 21778213 in words is "twenty-one million, seven hundred seventy-eight thousand, two hundred thirteen".