21539978 has 16 divisors (see below), whose sum is σ = 33686016. Its totient is φ = 10318320.

The previous prime is 21539971. The next prime is 21539993. The reversal of 21539978 is 87993512.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21539971) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4700 + ... + 8072.

It is an arithmetic number, because the mean of its divisors is an integer number (2105376).

Almost surely, 2^21539978 is an apocalyptic number.

21539978 is a deficient number, since it is larger than the sum of its proper divisors (12146038).

21539978 is a wasteful number, since it uses less digits than its factorization.

21539978 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3509.

The product of its digits is 136080, while the sum is 44.

The square root of 21539978 is about 4641.1181842310. The cubic root of 21539978 is about 278.2371301100.

The spelling of 21539978 in words is "twenty-one million, five hundred thirty-nine thousand, nine hundred seventy-eight".