21609991 has 4 divisors (see below), whose sum is σ = 23272312. Its totient is φ = 19947672.

The previous prime is 21609977. The next prime is 21610003. The reversal of 21609991 is 19990612.

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 21609991 - 2¹⁵ = 21577223 is a prime.

It is a super-2 number, since 2×21609991² = 933983422040162, which contains 22 as substring.

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 21609991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21609391) 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, 831141 + ... + 831166.

It is an arithmetic number, because the mean of its divisors is an integer number (5818078).

Almost surely, 2^21609991 is an apocalyptic number.

21609991 is a deficient number, since it is larger than the sum of its proper divisors (1662321).

21609991 is a wasteful number, since it uses less digits than its factorization.

21609991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1662320.

The product of its (nonzero) digits is 8748, while the sum is 37.

The square root of 21609991 is about 4648.6547516459. The cubic root of 21609991 is about 278.5382623925.

The spelling of 21609991 in words is "twenty-one million, six hundred nine thousand, nine hundred ninety-one".