21665191 has 4 divisors (see below), whose sum is σ = 22939632. Its totient is φ = 20390752.

The previous prime is 21665183. The next prime is 21665201. The reversal of 21665191 is 19156612.

It is a semiprime because it is the product of two primes.

It is not a de Polignac number, because 21665191 - 2³ = 21665183 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21665101) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 637195 + ... + 637228.

It is an arithmetic number, because the mean of its divisors is an integer number (5734908).

Almost surely, 2^21665191 is an apocalyptic number.

21665191 is a deficient number, since it is larger than the sum of its proper divisors (1274441).

21665191 is a wasteful number, since it uses less digits than its factorization.

21665191 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1274440.

The product of its digits is 3240, while the sum is 31.

The square root of 21665191 is about 4654.5881665299. The cubic root of 21665191 is about 278.7752243792.

The spelling of 21665191 in words is "twenty-one million, six hundred sixty-five thousand, one hundred ninety-one".