21723251 has 16 divisors (see below), whose sum is σ = 25180800. Its totient is φ = 18687240.

The previous prime is 21723239. The next prime is 21723253. The reversal of 21723251 is 15232712.

It is a happy number.

It is not a de Polignac number, because 21723251 - 2⁶ = 21723187 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21723253) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 24860 + ... + 25718.

It is an arithmetic number, because the mean of its divisors is an integer number (1573800).

Almost surely, 2^21723251 is an apocalyptic number.

21723251 is a deficient number, since it is larger than the sum of its proper divisors (3457549).

21723251 is an equidigital number, since it uses as much as digits as its factorization.

21723251 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 911 (or 889 counting only the distinct ones).

The product of its digits is 840, while the sum is 23.

The square root of 21723251 is about 4660.8208504511. The cubic root of 21723251 is about 279.0240298244.

Adding to 21723251 its reverse (15232712), we get a palindrome (36955963).

The spelling of 21723251 in words is "twenty-one million, seven hundred twenty-three thousand, two hundred fifty-one".