21781819 has 4 divisors (see below), whose sum is σ = 22138960. Its totient is φ = 21424680.

The previous prime is 21781807. The next prime is 21781829. The reversal of 21781819 is 91818712.

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 21781819 - 2¹⁵ = 21749051 is a prime.

It is a super-2 number, since 2×21781819² = 948895277897522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21781829) 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, 178479 + ... + 178600.

It is an arithmetic number, because the mean of its divisors is an integer number (5534740).

Almost surely, 2^21781819 is an apocalyptic number.

21781819 is a deficient number, since it is larger than the sum of its proper divisors (357141).

21781819 is an equidigital number, since it uses as much as digits as its factorization.

21781819 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 357140.

The product of its digits is 8064, while the sum is 37.

The square root of 21781819 is about 4667.0996346768. The cubic root of 21781819 is about 279.2745634562.

The spelling of 21781819 in words is "twenty-one million, seven hundred eighty-one thousand, eight hundred nineteen".