21501221 has 4 divisors (see below), whose sum is σ = 24572832. Its totient is φ = 18429612.

The previous prime is 21501211. The next prime is 21501229. The reversal of 21501221 is 12210512.

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 cyclic number.

It is not a de Polignac number, because 21501221 - 2¹⁸ = 21239077 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21501196 and 21501205.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21501229) 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, 1535795 + ... + 1535808.

It is an arithmetic number, because the mean of its divisors is an integer number (6143208).

Almost surely, 2^21501221 is an apocalyptic number.

It is an amenable number.

21501221 is a deficient number, since it is larger than the sum of its proper divisors (3071611).

21501221 is an equidigital number, since it uses as much as digits as its factorization.

21501221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3071610.

The product of its (nonzero) digits is 40, while the sum is 14.

The square root of 21501221 is about 4636.9409096947. The cubic root of 21501221 is about 278.0701520594.

Adding to 21501221 its reverse (12210512), we get a palindrome (33711733).

The spelling of 21501221 in words is "twenty-one million, five hundred one thousand, two hundred twenty-one".