21656723 has 12 divisors (see below), whose sum is σ = 23985384. Its totient is φ = 19392000.

The previous prime is 21656711. The next prime is 21656741. The reversal of 21656723 is 32765612.

It is not a de Polignac number, because 21656723 - 2²⁴ = 4879507 is a prime.

It is an alternating number because its digits alternate between even and odd.

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 21656723.

It is not an unprimeable number, because it can be changed into a prime (21656743) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 112115 + ... + 112307.

It is an arithmetic number, because the mean of its divisors is an integer number (1998782).

Almost surely, 2^21656723 is an apocalyptic number.

21656723 is a deficient number, since it is larger than the sum of its proper divisors (2328661).

21656723 is a wasteful number, since it uses less digits than its factorization.

21656723 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 406 (or 305 counting only the distinct ones).

The product of its digits is 15120, while the sum is 32.

The square root of 21656723 is about 4653.6784375373. The cubic root of 21656723 is about 278.7388991940.

Subtracting 21656723 from its reverse (32765612), we obtain a square (11108889 = 3333²).

It can be divided in two parts, 2165 and 6723, that added together give a palindrome (8888).

The spelling of 21656723 in words is "twenty-one million, six hundred fifty-six thousand, seven hundred twenty-three".