20991587 has 4 divisors (see below), whose sum is σ = 21016296. Its totient is φ = 20966880.

The previous prime is 20991577. The next prime is 20991601. The reversal of 20991587 is 78519902.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 78519902 = 2 ⋅39259951.

It is a cyclic number.

It is not a de Polignac number, because 20991587 - 2¹⁴ = 20975203 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20991517) 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, 11033 + ... + 12794.

It is an arithmetic number, because the mean of its divisors is an integer number (5254074).

Almost surely, 2^20991587 is an apocalyptic number.

20991587 is a deficient number, since it is larger than the sum of its proper divisors (24709).

20991587 is an equidigital number, since it uses as much as digits as its factorization.

20991587 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 24708.

The product of its (nonzero) digits is 45360, while the sum is 41.

The square root of 20991587 is about 4581.6576694467. The cubic root of 20991587 is about 275.8555701312.

The spelling of 20991587 in words is "twenty million, nine hundred ninety-one thousand, five hundred eighty-seven".