21204111991 has 2 divisors, whose sum is σ = 21204111992. Its totient is φ = 21204111990.

The previous prime is 21204111979. The next prime is 21204112001. The reversal of 21204111991 is 19911140212.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21204111991 - 2⁹ = 21204111479 is a prime.

It is a super-2 number, since 2×21204111991² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21204111911) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10602055995 + 10602055996.

It is an arithmetic number, because the mean of its divisors is an integer number (10602055996).

Almost surely, 2^21204111991 is an apocalyptic number.

21204111991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21204111991 is an equidigital number, since it uses as much as digits as its factorization.

21204111991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1296, while the sum is 31.

The spelling of 21204111991 in words is "twenty-one billion, two hundred four million, one hundred eleven thousand, nine hundred ninety-one".