21816421 has 16 divisors (see below), whose sum is σ = 24700608. Its totient is φ = 19253520.

The previous prime is 21816407. The next prime is 21816451. The reversal of 21816421 is 12461812.

It is not a de Polignac number, because 21816421 - 2⁷ = 21816293 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21816451) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 49026 + ... + 49468.

It is an arithmetic number, because the mean of its divisors is an integer number (1543788).

Almost surely, 2^21816421 is an apocalyptic number.

It is an amenable number.

21816421 is a deficient number, since it is larger than the sum of its proper divisors (2884187).

21816421 is an equidigital number, since it uses as much as digits as its factorization.

21816421 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 513 (or 491 counting only the distinct ones).

The product of its digits is 768, while the sum is 25.

The square root of 21816421 is about 4670.8051768405. The cubic root of 21816421 is about 279.4223678540.

The spelling of 21816421 in words is "twenty-one million, eight hundred sixteen thousand, four hundred twenty-one".