21157723 has 4 divisors (see below), whose sum is σ = 22077648. Its totient is φ = 20237800.

The previous prime is 21157699. The next prime is 21157739. The reversal of 21157723 is 32775112.

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 21157723 - 2¹⁷ = 21026651 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21157691 and 21157700.

It is not an unprimeable number, because it can be changed into a prime (21157753) 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, 459928 + ... + 459973.

It is an arithmetic number, because the mean of its divisors is an integer number (5519412).

Almost surely, 2^21157723 is an apocalyptic number.

21157723 is a gapful number since it is divisible by the number (23) formed by its first and last digit.

21157723 is a deficient number, since it is larger than the sum of its proper divisors (919925).

21157723 is an equidigital number, since it uses as much as digits as its factorization.

21157723 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 919924.

The product of its digits is 2940, while the sum is 28.

The square root of 21157723 is about 4599.7524933414. The cubic root of 21157723 is about 276.5814032192.

It can be divided in two parts, 211577 and 23, that added together give a square (211600 = 460²).

The spelling of 21157723 in words is "twenty-one million, one hundred fifty-seven thousand, seven hundred twenty-three".