20871991 has 6 divisors (see below), whose sum is σ = 24279720. Its totient is φ = 17890236.

The previous prime is 20871973. The next prime is 20872003. The reversal of 20871991 is 19917802.

It is not a de Polignac number, because 20871991 - 2⁷ = 20871863 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 20871991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20871491) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 212931 + ... + 213028.

It is an arithmetic number, because the mean of its divisors is an integer number (4046620).

Almost surely, 2^20871991 is an apocalyptic number.

20871991 is a deficient number, since it is larger than the sum of its proper divisors (3407729).

20871991 is an equidigital number, since it uses as much as digits as its factorization.

20871991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 425973 (or 425966 counting only the distinct ones).

The product of its (nonzero) digits is 9072, while the sum is 37.

The square root of 20871991 is about 4568.5874184479. The cubic root of 20871991 is about 275.3306919853.

It can be divided in two parts, 208 and 71991, that multiplied together give a triangular number (14974128 = T₅₄₇₂).

The spelling of 20871991 in words is "twenty million, eight hundred seventy-one thousand, nine hundred ninety-one".