21505123 has 4 divisors (see below), whose sum is σ = 21857728. Its totient is φ = 21152520.

The previous prime is 21505109. The next prime is 21505129. The reversal of 21505123 is 32150512.

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 21505123 - 2¹⁷ = 21374051 is a prime.

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 21505123.

It is not an unprimeable number, because it can be changed into a prime (21505129) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 176211 + ... + 176332.

It is an arithmetic number, because the mean of its divisors is an integer number (5464432).

Almost surely, 2^21505123 is an apocalyptic number.

21505123 is a deficient number, since it is larger than the sum of its proper divisors (352605).

21505123 is an equidigital number, since it uses as much as digits as its factorization.

21505123 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 352604.

The product of its (nonzero) digits is 300, while the sum is 19.

The square root of 21505123 is about 4637.3616421409. The cubic root of 21505123 is about 278.0869722531.

Adding to 21505123 its reverse (32150512), we get a palindrome (53655635).

The spelling of 21505123 in words is "twenty-one million, five hundred five thousand, one hundred twenty-three".