21001000991 has 2 divisors, whose sum is σ = 21001000992. Its totient is φ = 21001000990.

The previous prime is 21001000927. The next prime is 21001001009. The reversal of 21001000991 is 19900010012.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21001000991 - 2⁶ = 21001000927 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21001000901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10500500495 + 10500500496.

It is an arithmetic number, because the mean of its divisors is an integer number (10500500496).

Almost surely, 2^21001000991 is an apocalyptic number.

21001000991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21001000991 is an equidigital number, since it uses as much as digits as its factorization.

21001000991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 162, while the sum is 23.

The spelling of 21001000991 in words is "twenty-one billion, one million, nine hundred ninety-one", and thus it is an aban number.