21171991 has 2 divisors, whose sum is σ = 21171992. Its totient is φ = 21171990.

The previous prime is 21171989. The next prime is 21172003. The reversal of 21171991 is 19917112.

It is a happy number.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21171991 - 2¹ = 21171989 is a prime.

Together with 21171989, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21171911) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10585995 + 10585996.

It is an arithmetic number, because the mean of its divisors is an integer number (10585996).

Almost surely, 2^21171991 is an apocalyptic number.

21171991 is the 2460-th centered heptagonal number.

21171991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21171991 is an equidigital number, since it uses as much as digits as its factorization.

21171991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1134, while the sum is 31.

The square root of 21171991 is about 4601.3031849684. The cubic root of 21171991 is about 276.6435614001.

The spelling of 21171991 in words is "twenty-one million, one hundred seventy-one thousand, nine hundred ninety-one".