21661823 has 4 divisors (see below), whose sum is σ = 21671328. Its totient is φ = 21652320.

The previous prime is 21661813. The next prime is 21661831. The reversal of 21661823 is 32816612.

It is a semiprime because it is the product of two primes, 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 21661823 - 2⁴ = 21661807 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21661813) 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, 938 + ... + 6648.

It is an arithmetic number, because the mean of its divisors is an integer number (5417832).

Almost surely, 2^21661823 is an apocalyptic number.

21661823 is a deficient number, since it is larger than the sum of its proper divisors (9505).

21661823 is an equidigital number, since it uses as much as digits as its factorization.

21661823 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9504.

The product of its digits is 3456, while the sum is 29.

The square root of 21661823 is about 4654.2263589129. The cubic root of 21661823 is about 278.7607778012.

The spelling of 21661823 in words is "twenty-one million, six hundred sixty-one thousand, eight hundred twenty-three".