20556247991 has 4 divisors (see below), whose sum is σ = 21638155800. Its totient is φ = 19474340184.

The previous prime is 20556247967. The next prime is 20556248017. The reversal of 20556247991 is 19974265502.

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 20556247991 - 2¹⁰ = 20556246967 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 (20556247951) 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, 540953876 + ... + 540953913.

It is an arithmetic number, because the mean of its divisors is an integer number (5409538950).

Almost surely, 2^20556247991 is an apocalyptic number.

20556247991 is a deficient number, since it is larger than the sum of its proper divisors (1081907809).

20556247991 is a wasteful number, since it uses less digits than its factorization.

20556247991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1081907808.

The product of its (nonzero) digits is 1360800, while the sum is 50.

The spelling of 20556247991 in words is "twenty billion, five hundred fifty-six million, two hundred forty-seven thousand, nine hundred ninety-one".