21001991 has 2 divisors, whose sum is σ = 21001992. Its totient is φ = 21001990.

The previous prime is 21001979. The next prime is 21002039. The reversal of 21001991 is 19910012.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21001991 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21001961) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10500995 + 10500996.

It is an arithmetic number, because the mean of its divisors is an integer number (10500996).

Almost surely, 2^21001991 is an apocalyptic number.

21001991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21001991 is an equidigital number, since it uses as much as digits as its factorization.

21001991 is an odious number, because the sum of its binary digits is odd.

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

The square root of 21001991 is about 4582.7929257168. The cubic root of 21001991 is about 275.9011364388.

The spelling of 21001991 in words is "twenty-one million, one thousand, nine hundred ninety-one".