21821643 has 24 divisors (see below), whose sum is σ = 33087600. Its totient is φ = 14370048.

The previous prime is 21821629. The next prime is 21821647. The reversal of 21821643 is 34612812.

21821643 is a `hidden beast` number, since 2 + 18 + 2 + 1 + 643 = 666.

It is a de Polignac number, because none of the positive numbers 2^k-21821643 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21821647) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 21123 + ... + 22131.

It is an arithmetic number, because the mean of its divisors is an integer number (1378650).

Almost surely, 2^21821643 is an apocalyptic number.

21821643 is a deficient number, since it is larger than the sum of its proper divisors (11265957).

21821643 is an equidigital number, since it uses as much as digits as its factorization.

21821643 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1113 (or 1101 counting only the distinct ones).

The product of its digits is 2304, while the sum is 27.

The square root of 21821643 is about 4671.3641476554. The cubic root of 21821643 is about 279.4446603464. Note that the first 3 decimals are identical.

It can be divided in two parts, 218 and 21643, that multiplied together give a palindrome (4718174).

The spelling of 21821643 in words is "twenty-one million, eight hundred twenty-one thousand, six hundred forty-three".