21878491 has 2 divisors, whose sum is σ = 21878492. Its totient is φ = 21878490.

The previous prime is 21878459. The next prime is 21878531. The reversal of 21878491 is 19487812.

It is a happy number.

It is an a-pointer prime, because the next prime (21878531) can be obtained adding 21878491 to its sum of digits (40).

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21878491 - 2⁵ = 21878459 is a prime.

It is not a weakly prime, because it can be changed into another prime (21878401) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10939245 + 10939246.

It is an arithmetic number, because the mean of its divisors is an integer number (10939246).

Almost surely, 2^21878491 is an apocalyptic number.

21878491 is a deficient number, since it is larger than the sum of its proper divisors (1).

21878491 is an equidigital number, since it uses as much as digits as its factorization.

21878491 is an evil number, because the sum of its binary digits is even.

The product of its digits is 32256, while the sum is 40.

The square root of 21878491 is about 4677.4449221771. Note that the first 3 decimals coincide. The cubic root of 21878491 is about 279.6871122265.

The spelling of 21878491 in words is "twenty-one million, eight hundred seventy-eight thousand, four hundred ninety-one".