21181621 has 4 divisors (see below), whose sum is σ = 21192192. Its totient is φ = 21171052.

The previous prime is 21181603. The next prime is 21181627. The reversal of 21181621 is 12618112.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21181621 - 2⁹ = 21181109 is a prime.

It is a super-2 number, since 2×21181621² = 897322136375282, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21181592 and 21181601.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21181627) 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, 1255 + ... + 6628.

It is an arithmetic number, because the mean of its divisors is an integer number (5298048).

Almost surely, 2^21181621 is an apocalyptic number.

It is an amenable number.

21181621 is a deficient number, since it is larger than the sum of its proper divisors (10571).

21181621 is an equidigital number, since it uses as much as digits as its factorization.

21181621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10570.

The product of its digits is 192, while the sum is 22.

The square root of 21181621 is about 4602.3495086749. The cubic root of 21181621 is about 276.6854984682.

Adding to 21181621 its reverse (12618112), we get a palindrome (33799733).

The spelling of 21181621 in words is "twenty-one million, one hundred eighty-one thousand, six hundred twenty-one".