21085781 has 4 divisors (see below), whose sum is σ = 21576192. Its totient is φ = 20595372.

The previous prime is 21085777. The next prime is 21085787. The reversal of 21085781 is 18758012.

It is a happy number.

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 a cyclic number.

It is not a de Polignac number, because 21085781 - 2² = 21085777 is a prime.

It is a super-2 number, since 2×21085781² = 889220320759922, 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 (21085787) 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, 245141 + ... + 245226.

It is an arithmetic number, because the mean of its divisors is an integer number (5394048).

Almost surely, 2^21085781 is an apocalyptic number.

It is an amenable number.

21085781 is a deficient number, since it is larger than the sum of its proper divisors (490411).

21085781 is an equidigital number, since it uses as much as digits as its factorization.

21085781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 490410.

The product of its (nonzero) digits is 4480, while the sum is 32.

The square root of 21085781 is about 4591.9256309309. The cubic root of 21085781 is about 276.2675632930.

The spelling of 21085781 in words is "twenty-one million, eighty-five thousand, seven hundred eighty-one".