21505423 has 4 divisors (see below), whose sum is σ = 21529080. Its totient is φ = 21481768.

The previous prime is 21505417. The next prime is 21505447. The reversal of 21505423 is 32450512.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21505423 - 2¹⁷ = 21374351 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21505394 and 21505403.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21503423) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10408 + ... + 12301.

It is an arithmetic number, because the mean of its divisors is an integer number (5382270).

Almost surely, 2^21505423 is an apocalyptic number.

21505423 is a deficient number, since it is larger than the sum of its proper divisors (23657).

21505423 is an equidigital number, since it uses as much as digits as its factorization.

21505423 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 23656.

The product of its (nonzero) digits is 1200, while the sum is 22.

The square root of 21505423 is about 4637.3939880066. The cubic root of 21505423 is about 278.0882653667.

Adding to 21505423 its reverse (32450512), we get a palindrome (53955935).

The spelling of 21505423 in words is "twenty-one million, five hundred five thousand, four hundred twenty-three".