21156781 has 4 divisors (see below), whose sum is σ = 21168000. Its totient is φ = 21145564.

The previous prime is 21156769. The next prime is 21156809. The reversal of 21156781 is 18765112.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21156781 - 2⁵ = 21156749 is a prime.

It is a super-2 number, since 2×21156781² = 895218764563922, 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 (21156701) 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, 2011 + ... + 6808.

It is an arithmetic number, because the mean of its divisors is an integer number (5292000).

Almost surely, 2^21156781 is an apocalyptic number.

It is an amenable number.

21156781 is a deficient number, since it is larger than the sum of its proper divisors (11219).

21156781 is an equidigital number, since it uses as much as digits as its factorization.

21156781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11218.

The product of its digits is 3360, while the sum is 31.

The square root of 21156781 is about 4599.6500953877. The cubic root of 21156781 is about 276.5772984368.

The spelling of 21156781 in words is "twenty-one million, one hundred fifty-six thousand, seven hundred eighty-one".