21978161 has 12 divisors (see below), whose sum is σ = 23588652. Its totient is φ = 20471808.

The previous prime is 21978139. The next prime is 21978167. The reversal of 21978161 is 16187912.

It can be written as a sum of positive squares in 6 ways, for example, as 309136 + 21669025 = 556^2 + 4655^2 .

It is not a de Polignac number, because 21978161 - 2⁶ = 21978097 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21978167) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 32321 + ... + 32993.

It is an arithmetic number, because the mean of its divisors is an integer number (1965721).

Almost surely, 2^21978161 is an apocalyptic number.

It is an amenable number.

21978161 is a deficient number, since it is larger than the sum of its proper divisors (1610491).

21978161 is a wasteful number, since it uses less digits than its factorization.

21978161 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 820 (or 803 counting only the distinct ones).

The product of its digits is 6048, while the sum is 35.

The square root of 21978161 is about 4688.0871365622. The cubic root of 21978161 is about 280.1111845856. Note that the first 4 decimals are identical.

The spelling of 21978161 in words is "twenty-one million, nine hundred seventy-eight thousand, one hundred sixty-one".