20991691 has 8 divisors (see below), whose sum is σ = 24639200. Its totient is φ = 17506368.

The previous prime is 20991653. The next prime is 20991721. The reversal of 20991691 is 19619902.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20991691 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (37).

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 20991691.

It is not an unprimeable number, because it can be changed into a prime (20991601) 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 7 ways as a sum of consecutive naturals, for example, 40266 + ... + 40783.

It is an arithmetic number, because the mean of its divisors is an integer number (3079900).

Almost surely, 2^20991691 is an apocalyptic number.

20991691 is a deficient number, since it is larger than the sum of its proper divisors (3647509).

20991691 is an equidigital number, since it uses as much as digits as its factorization.

20991691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 81093.

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

The square root of 20991691 is about 4581.6690190366. The cubic root of 20991691 is about 275.8560256936.

The spelling of 20991691 in words is "twenty million, nine hundred ninety-one thousand, six hundred ninety-one".