21405681 has 16 divisors (see below), whose sum is σ = 34595520. Its totient is φ = 12972960.

The previous prime is 21405679. The next prime is 21405691. The reversal of 21405681 is 18650412.

It is not a de Polignac number, because 21405681 - 2¹ = 21405679 is a prime.

It is a super-2 number, since 2×21405681² = 916406358147522, which contains 22 as substring.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21405611) 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, 35740 + ... + 36333.

It is an arithmetic number, because the mean of its divisors is an integer number (2162220).

Almost surely, 2^21405681 is an apocalyptic number.

It is an amenable number.

21405681 is a deficient number, since it is larger than the sum of its proper divisors (13189839).

21405681 is a wasteful number, since it uses less digits than its factorization.

21405681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 72093 (or 72087 counting only the distinct ones).

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

The square root of 21405681 is about 4626.6273893626. The cubic root of 21405681 is about 277.6576751840.

The spelling of 21405681 in words is "twenty-one million, four hundred five thousand, six hundred eighty-one".