21215621 has 4 divisors (see below), whose sum is σ = 24246432. Its totient is φ = 18184812.

The previous prime is 21215611. The next prime is 21215627. The reversal of 21215621 is 12651212.

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 21215621 - 2²² = 17021317 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21215593 and 21215602.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21215627) 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, 1515395 + ... + 1515408.

It is an arithmetic number, because the mean of its divisors is an integer number (6061608).

Almost surely, 2^21215621 is an apocalyptic number.

It is an amenable number.

21215621 is a deficient number, since it is larger than the sum of its proper divisors (3030811).

21215621 is an equidigital number, since it uses as much as digits as its factorization.

21215621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3030810.

The product of its digits is 240, while the sum is 20.

The square root of 21215621 is about 4606.0417931235. The cubic root of 21215621 is about 276.8334613020.

Adding to 21215621 its reverse (12651212), we get a palindrome (33866833).

The spelling of 21215621 in words is "twenty-one million, two hundred fifteen thousand, six hundred twenty-one".