21128123 has 4 divisors (see below), whose sum is σ = 21137640. Its totient is φ = 21118608.

The previous prime is 21128119. The next prime is 21128131. The reversal of 21128123 is 32182112.

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 21128123 - 2² = 21128119 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21128095 and 21128104.

It is not an unprimeable number, because it can be changed into a prime (21128183) 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, 536 + ... + 6522.

It is an arithmetic number, because the mean of its divisors is an integer number (5284410).

Almost surely, 2^21128123 is an apocalyptic number.

21128123 is a deficient number, since it is larger than the sum of its proper divisors (9517).

21128123 is an equidigital number, since it uses as much as digits as its factorization.

21128123 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9516.

The product of its digits is 192, while the sum is 20.

The square root of 21128123 is about 4596.5338027692. The cubic root of 21128123 is about 276.4523623904.

The spelling of 21128123 in words is "twenty-one million, one hundred twenty-eight thousand, one hundred twenty-three".