21669991 has 4 divisors (see below), whose sum is σ = 24765712. Its totient is φ = 18574272.

The previous prime is 21669959. The next prime is 21670003. The reversal of 21669991 is 19996612.

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

It is a cyclic number.

It is not a de Polignac number, because 21669991 - 2⁵ = 21669959 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21669191) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1547850 + ... + 1547863.

It is an arithmetic number, because the mean of its divisors is an integer number (6191428).

Almost surely, 2^21669991 is an apocalyptic number.

21669991 is a deficient number, since it is larger than the sum of its proper divisors (3095721).

21669991 is an equidigital number, since it uses as much as digits as its factorization.

21669991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3095720.

The product of its digits is 52488, while the sum is 43.

The square root of 21669991 is about 4655.1037582421. The cubic root of 21669991 is about 278.7958107392.

It can be divided in two parts, 216 and 69991, that added together give a palindrome (70207).

The spelling of 21669991 in words is "twenty-one million, six hundred sixty-nine thousand, nine hundred ninety-one".