21531664 has 40 divisors (see below), whose sum is σ = 52014528. Its totient is φ = 8388480.

The previous prime is 21531649. The next prime is 21531667. The reversal of 21531664 is 46613512.

It is a super-2 number, since 2×21531664² = 927225109217792, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (28).

It is not an unprimeable number, because it can be changed into a prime (21531667) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 7507 + ... + 9970.

Almost surely, 2^21531664 is an apocalyptic number.

It is an amenable number.

21531664 is an abundant number, since it is smaller than the sum of its proper divisors (30482864).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21531664 is a wasteful number, since it uses less digits than its factorization.

21531664 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 17503 (or 17497 counting only the distinct ones).

The product of its digits is 4320, while the sum is 28.

The square root of 21531664 is about 4640.2224084628. Note that the first 3 decimals coincide. The cubic root of 21531664 is about 278.2013275172.

The spelling of 21531664 in words is "twenty-one million, five hundred thirty-one thousand, six hundred sixty-four".