21978181 has 2 divisors, whose sum is σ = 21978182. Its totient is φ = 21978180.

The previous prime is 21978169. The next prime is 21978217. The reversal of 21978181 is 18187912.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13032100 + 8946081 = 3610^2 + 2991^2 .

It is a cyclic number.

It is not a de Polignac number, because 21978181 - 2¹⁷ = 21847109 is a prime.

It is a super-2 number, since 2×21978181² = 966080880137522, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21978181.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21978581) 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 as a sum of consecutive naturals, namely, 10989090 + 10989091.

It is an arithmetic number, because the mean of its divisors is an integer number (10989091).

Almost surely, 2^21978181 is an apocalyptic number.

It is an amenable number.

21978181 is a deficient number, since it is larger than the sum of its proper divisors (1).

21978181 is an equidigital number, since it uses as much as digits as its factorization.

21978181 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 8064, while the sum is 37.

The square root of 21978181 is about 4688.0892696279. The cubic root of 21978181 is about 280.1112695521. Note that the first 3 decimals are identical.

It can be divided in two parts, 219 and 78181, that added together give a square (78400 = 280²).

The spelling of 21978181 in words is "twenty-one million, nine hundred seventy-eight thousand, one hundred eighty-one".