21009891197 has 2 divisors, whose sum is σ = 21009891198. Its totient is φ = 21009891196.

The previous prime is 21009891179. The next prime is 21009891223. The reversal of 21009891197 is 79119890012.

Together with previous prime (21009891179) it forms an Ormiston pair, because they use the same digits, order apart.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 10539280921 + 10470610276 = 102661^2 + 102326^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21009891197 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21009891997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504945598 + 10504945599.

It is an arithmetic number, because the mean of its divisors is an integer number (10504945599).

Almost surely, 2^21009891197 is an apocalyptic number.

It is an amenable number.

21009891197 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009891197 is an equidigital number, since it uses as much as digits as its factorization.

21009891197 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 81648, while the sum is 47.

The spelling of 21009891197 in words is "twenty-one billion, nine million, eight hundred ninety-one thousand, one hundred ninety-seven".