21187971 has 12 divisors (see below), whose sum is σ = 34977072. Its totient is φ = 12107376.

The previous prime is 21187961. The next prime is 21187979. The reversal of 21187971 is 17978112.

It is not a de Polignac number, because 21187971 - 2⁷ = 21187843 is a prime.

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

It is not an unprimeable number, because it can be changed into a prime (21187979) 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 in 11 ways as a sum of consecutive naturals, for example, 168096 + ... + 168221.

It is an arithmetic number, because the mean of its divisors is an integer number (2914756).

Almost surely, 2^21187971 is an apocalyptic number.

21187971 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21187971 is a deficient number, since it is larger than the sum of its proper divisors (13789101).

21187971 is a wasteful number, since it uses less digits than its factorization.

21187971 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 336330 (or 336327 counting only the distinct ones).

The product of its digits is 7056, while the sum is 36.

The square root of 21187971 is about 4603.0393220132. The cubic root of 21187971 is about 276.7131447214.

The spelling of 21187971 in words is "twenty-one million, one hundred eighty-seven thousand, nine hundred seventy-one".