21978251 has 2 divisors, whose sum is σ = 21978252. Its totient is φ = 21978250.

The previous prime is 21978247. The next prime is 21978293. The reversal of 21978251 is 15287912.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21978251 - 2² = 21978247 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21978241) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10989125 + 10989126.

It is an arithmetic number, because the mean of its divisors is an integer number (10989126).

Almost surely, 2^21978251 is an apocalyptic number.

21978251 is a deficient number, since it is larger than the sum of its proper divisors (1).

21978251 is an equidigital number, since it uses as much as digits as its factorization.

21978251 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10080, while the sum is 35.

The square root of 21978251 is about 4688.0967353501. The cubic root of 21978251 is about 280.1115669344. Note that the first 3 decimals are identical.

The spelling of 21978251 in words is "twenty-one million, nine hundred seventy-eight thousand, two hundred fifty-one".