21378781 has 6 divisors (see below), whose sum is σ = 22563582. Its totient is φ = 20253240.

The previous prime is 21378767. The next prime is 21378817. The reversal of 21378781 is 18787312.

It can be written as a sum of positive squares in only one way, i.e., 16687225 + 4691556 = 4085^2 + 2166^2 .

It is not a de Polignac number, because 21378781 - 2⁷ = 21378653 is a prime.

It is a super-2 number, since 2×21378781² = 914104554091922, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21378781.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21378701) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 29250 + ... + 29971.

It is an arithmetic number, because the mean of its divisors is an integer number (3760597).

Almost surely, 2^21378781 is an apocalyptic number.

It is an amenable number.

21378781 is a deficient number, since it is larger than the sum of its proper divisors (1184801).

21378781 is an equidigital number, since it uses as much as digits as its factorization.

21378781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 59259 (or 59240 counting only the distinct ones).

The product of its digits is 18816, while the sum is 37.

The square root of 21378781 is about 4623.7193902745. The cubic root of 21378781 is about 277.5413178745.

The spelling of 21378781 in words is "twenty-one million, three hundred seventy-eight thousand, seven hundred eighty-one".