21171989 has 2 divisors, whose sum is σ = 21171990. Its totient is φ = 21171988.

The previous prime is 21171961. The next prime is 21171991. The reversal of 21171989 is 98917112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21003889 + 168100 = 4583^2 + 410^2 .

It is a cyclic number.

It is not a de Polignac number, because 21171989 - 2¹⁶ = 21106453 is a prime.

It is a super-2 number, since 2×21171989² = 896506236432242, which contains 22 as substring.

Together with 21171991, 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 (21171929) 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, 10585994 + 10585995.

It is an arithmetic number, because the mean of its divisors is an integer number (10585995).

Almost surely, 2^21171989 is an apocalyptic number.

It is an amenable number.

21171989 is a deficient number, since it is larger than the sum of its proper divisors (1).

21171989 is an equidigital number, since it uses as much as digits as its factorization.

21171989 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 9072, while the sum is 38.

The square root of 21171989 is about 4601.3029676386. The cubic root of 21171989 is about 276.6435526891.

The spelling of 21171989 in words is "twenty-one million, one hundred seventy-one thousand, nine hundred eighty-nine".