21661251 has 4 divisors (see below), whose sum is σ = 28881672. Its totient is φ = 14440832.

The previous prime is 21661223. The next prime is 21661253. The reversal of 21661251 is 15216612.

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 21661251 - 2¹¹ = 21659203 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21661253) 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, 3610206 + ... + 3610211.

It is an arithmetic number, because the mean of its divisors is an integer number (7220418).

Almost surely, 2^21661251 is an apocalyptic number.

21661251 is a deficient number, since it is larger than the sum of its proper divisors (7220421).

21661251 is an equidigital number, since it uses as much as digits as its factorization.

21661251 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7220420.

The product of its digits is 720, while the sum is 24.

The square root of 21661251 is about 4654.1649089821. The cubic root of 21661251 is about 278.7583241363.

Adding to 21661251 its reverse (15216612), we get a palindrome (36877863).

It can be divided in two parts, 21661 and 251, that added together give a palindrome (21912).

The spelling of 21661251 in words is "twenty-one million, six hundred sixty-one thousand, two hundred fifty-one".