21681978 has 8 divisors (see below), whose sum is σ = 43363968. Its totient is φ = 7227324.

The previous prime is 21681977. The next prime is 21681991. The reversal of 21681978 is 87918612.

It is a sphenic number, since it is the product of 3 distinct primes.

21681978 is an admirable number.

It is not an unprimeable number, because it can be changed into a prime (21681971) 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, 1806826 + ... + 1806837.

It is an arithmetic number, because the mean of its divisors is an integer number (5420496).

Almost surely, 2^21681978 is an apocalyptic number.

21681978 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

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

21681978 is a wasteful number, since it uses less digits than its factorization.

21681978 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3613668.

The product of its digits is 48384, while the sum is 42.

The square root of 21681978 is about 4656.3910918221. The cubic root of 21681978 is about 278.8472076128.

The spelling of 21681978 in words is "twenty-one million, six hundred eighty-one thousand, nine hundred seventy-eight".