21665243 has 2 divisors, whose sum is σ = 21665244. Its totient is φ = 21665242.

The previous prime is 21665227. The next prime is 21665251. The reversal of 21665243 is 34256612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21665243 - 2⁴ = 21665227 is a prime.

It is a Sophie Germain prime.

It is not a weakly prime, because it can be changed into another prime (21665143) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10832621 + 10832622.

It is an arithmetic number, because the mean of its divisors is an integer number (10832622).

Almost surely, 2^21665243 is an apocalyptic number.

21665243 is a deficient number, since it is larger than the sum of its proper divisors (1).

21665243 is an equidigital number, since it uses as much as digits as its factorization.

21665243 is an evil number, because the sum of its binary digits is even.

The product of its digits is 8640, while the sum is 29.

The square root of 21665243 is about 4654.5937524128. The cubic root of 21665243 is about 278.7754474144.

The spelling of 21665243 in words is "twenty-one million, six hundred sixty-five thousand, two hundred forty-three".