20551991 has 4 divisors (see below), whose sum is σ = 20658672. Its totient is φ = 20445312.

The previous prime is 20551987. The next prime is 20552009. The reversal of 20551991 is 19915502.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19915502 = 2 ⋅9957751.

It is a cyclic number.

It is not a de Polignac number, because 20551991 - 2² = 20551987 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 (20551931) 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, 53051 + ... + 53436.

It is an arithmetic number, because the mean of its divisors is an integer number (5164668).

Almost surely, 2^20551991 is an apocalyptic number.

20551991 is a deficient number, since it is larger than the sum of its proper divisors (106681).

20551991 is a wasteful number, since it uses less digits than its factorization.

20551991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 106680.

The product of its (nonzero) digits is 4050, while the sum is 32.

The square root of 20551991 is about 4533.4303788632. The cubic root of 20551991 is about 273.9163569794.

The spelling of 20551991 in words is "twenty million, five hundred fifty-one thousand, nine hundred ninety-one".