21661221 has 4 divisors (see below), whose sum is σ = 28881632. Its totient is φ = 14440812.

The previous prime is 21661183. The next prime is 21661223. The reversal of 21661221 is 12216612.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a de Polignac number, because none of the positive numbers 2^k-21661221 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21661223) 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, 3610201 + ... + 3610206.

It is an arithmetic number, because the mean of its divisors is an integer number (7220408).

Almost surely, 2^21661221 is an apocalyptic number.

It is an amenable number.

21661221 is a deficient number, since it is larger than the sum of its proper divisors (7220411).

21661221 is an equidigital number, since it uses as much as digits as its factorization.

21661221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7220410.

The product of its digits is 288, while the sum is 21.

The square root of 21661221 is about 4654.1616860612. The cubic root of 21661221 is about 278.7581954463.

Adding to 21661221 its reverse (12216612), we get a palindrome (33877833).

The spelling of 21661221 in words is "twenty-one million, six hundred sixty-one thousand, two hundred twenty-one".