21878621 has 2 divisors, whose sum is σ = 21878622. Its totient is φ = 21878620.

The previous prime is 21878573. The next prime is 21878629. The reversal of 21878621 is 12687812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12355225 + 9523396 = 3515^2 + 3086^2 .

It is a cyclic number.

It is not a de Polignac number, because 21878621 - 2¹⁴ = 21862237 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21878629) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10939310 + 10939311.

It is an arithmetic number, because the mean of its divisors is an integer number (10939311).

Almost surely, 2^21878621 is an apocalyptic number.

It is an amenable number.

21878621 is a deficient number, since it is larger than the sum of its proper divisors (1).

21878621 is an equidigital number, since it uses as much as digits as its factorization.

21878621 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10752, while the sum is 35.

The square root of 21878621 is about 4677.4588186322. The cubic root of 21878621 is about 279.6876661839.

The spelling of 21878621 in words is "twenty-one million, eight hundred seventy-eight thousand, six hundred twenty-one".