21663391 has 2 divisors, whose sum is σ = 21663392. Its totient is φ = 21663390.

The previous prime is 21663373. The next prime is 21663403. The reversal of 21663391 is 19336612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21663391 - 2⁵ = 21663359 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21663091) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10831695 + 10831696.

It is an arithmetic number, because the mean of its divisors is an integer number (10831696).

Almost surely, 2^21663391 is an apocalyptic number.

21663391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21663391 is an equidigital number, since it uses as much as digits as its factorization.

21663391 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5832, while the sum is 31.

The square root of 21663391 is about 4654.3948049129. The cubic root of 21663391 is about 278.7675037103.

The spelling of 21663391 in words is "twenty-one million, six hundred sixty-three thousand, three hundred ninety-one".