21781816 has 16 divisors (see below), whose sum is σ = 46675440. Its totient is φ = 9335040.

The previous prime is 21781807. The next prime is 21781829. The reversal of 21781816 is 61818712.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 194425 + ... + 194536.

It is an arithmetic number, because the mean of its divisors is an integer number (2917215).

Almost surely, 2^21781816 is an apocalyptic number.

It is an amenable number.

21781816 is an abundant number, since it is smaller than the sum of its proper divisors (24893624).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21781816 is a wasteful number, since it uses less digits than its factorization.

21781816 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 388974 (or 388970 counting only the distinct ones).

The product of its digits is 5376, while the sum is 34.

The square root of 21781816 is about 4667.0993132780. The cubic root of 21781816 is about 279.2745506347.

The spelling of 21781816 in words is "twenty-one million, seven hundred eighty-one thousand, eight hundred sixteen".