21211221991 has 2 divisors, whose sum is σ = 21211221992. Its totient is φ = 21211221990.

The previous prime is 21211221961. The next prime is 21211222051. The reversal of 21211221991 is 19912211212.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21211221991 - 2⁵ = 21211221959 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21211221931) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10605610995 + 10605610996.

It is an arithmetic number, because the mean of its divisors is an integer number (10605610996).

Almost surely, 2^21211221991 is an apocalyptic number.

21211221991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21211221991 is an equidigital number, since it uses as much as digits as its factorization.

21211221991 is an odious number, because the sum of its binary digits is odd.

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

The spelling of 21211221991 in words is "twenty-one billion, two hundred eleven million, two hundred twenty-one thousand, nine hundred ninety-one".