21397991491 has 2 divisors, whose sum is σ = 21397991492. Its totient is φ = 21397991490.

The previous prime is 21397991453. The next prime is 21397991497. The reversal of 21397991491 is 19419979312.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21397991491 is a prime.

It is not a weakly prime, because it can be changed into another prime (21397991497) 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, 10698995745 + 10698995746.

It is an arithmetic number, because the mean of its divisors is an integer number (10698995746).

Almost surely, 2^21397991491 is an apocalyptic number.

21397991491 is a deficient number, since it is larger than the sum of its proper divisors (1).

21397991491 is an equidigital number, since it uses as much as digits as its factorization.

21397991491 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1102248, while the sum is 55.

The spelling of 21397991491 in words is "twenty-one billion, three hundred ninety-seven million, nine hundred ninety-one thousand, four hundred ninety-one".