21768391 has 2 divisors, whose sum is σ = 21768392. Its totient is φ = 21768390.

The previous prime is 21768361. The next prime is 21768407. The reversal of 21768391 is 19386712.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21768391 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21768391.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21768361) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10884195 + 10884196.

It is an arithmetic number, because the mean of its divisors is an integer number (10884196).

Almost surely, 2^21768391 is an apocalyptic number.

21768391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21768391 is an equidigital number, since it uses as much as digits as its factorization.

21768391 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 18144, while the sum is 37.

The square root of 21768391 is about 4665.6608320794. The cubic root of 21768391 is about 279.2171628366.

The spelling of 21768391 in words is "twenty-one million, seven hundred sixty-eight thousand, three hundred ninety-one".