21978217 has 2 divisors, whose sum is σ = 21978218. Its totient is φ = 21978216.

The previous prime is 21978181. The next prime is 21978227. The reversal of 21978217 is 71287912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21307456 + 670761 = 4616^2 + 819^2 .

It is a cyclic number.

It is not a de Polignac number, because 21978217 - 2¹¹ = 21976169 is a prime.

It is not a weakly prime, because it can be changed into another prime (21978227) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10989108 + 10989109.

It is an arithmetic number, because the mean of its divisors is an integer number (10989109).

Almost surely, 2^21978217 is an apocalyptic number.

It is an amenable number.

21978217 is a deficient number, since it is larger than the sum of its proper divisors (1).

21978217 is an equidigital number, since it uses as much as digits as its factorization.

21978217 is an evil number, because the sum of its binary digits is even.

The product of its digits is 14112, while the sum is 37.

The square root of 21978217 is about 4688.0931091436. The cubic root of 21978217 is about 280.1114224917. Note that the first 3 decimals are identical.

The spelling of 21978217 in words is "twenty-one million, nine hundred seventy-eight thousand, two hundred seventeen".