21865221 has 40 divisors (see below), whose sum is σ = 38139200. Its totient is φ = 12478536.

The previous prime is 21865211. The next prime is 21865229. The reversal of 21865221 is 12256812.

It is a happy number.

21865221 is a `hidden beast` number, since 2 + 1 + 8 + 652 + 2 + 1 = 666.

It is not a de Polignac number, because 21865221 - 2¹⁵ = 21832453 is a prime.

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

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21865229) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 27390 + ... + 28176.

It is an arithmetic number, because the mean of its divisors is an integer number (953480).

Almost surely, 2^21865221 is an apocalyptic number.

21865221 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21865221 is a deficient number, since it is larger than the sum of its proper divisors (16273979).

21865221 is an frugal number, since it uses more digits than its factorization.

21865221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 820 (or 797 counting only the distinct ones).

The product of its digits is 1920, while the sum is 27.

The square root of 21865221 is about 4676.0261975314. The cubic root of 21865221 is about 279.6305544166.

The spelling of 21865221 in words is "twenty-one million, eight hundred sixty-five thousand, two hundred twenty-one".