21681685 has 4 divisors (see below), whose sum is σ = 26018028. Its totient is φ = 17345344.

The previous prime is 21681683. The next prime is 21681689. The reversal of 21681685 is 58618612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 16695396 + 4986289 = 4086^2 + 2233^2 .

It is a cyclic number.

It is not a de Polignac number, because 21681685 - 2¹ = 21681683 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681683) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2168164 + ... + 2168173.

It is an arithmetic number, because the mean of its divisors is an integer number (6504507).

Almost surely, 2^21681685 is an apocalyptic number.

It is an amenable number.

21681685 is a deficient number, since it is larger than the sum of its proper divisors (4336343).

21681685 is an equidigital number, since it uses as much as digits as its factorization.

21681685 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4336342.

The product of its digits is 23040, while the sum is 37.

The square root of 21681685 is about 4656.3596295819. The cubic root of 21681685 is about 278.8459515374.

It can be divided in two parts, 21681 and 685, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21681685 in words is "twenty-one million, six hundred eighty-one thousand, six hundred eighty-five".