21518772787 has 4 divisors (see below), whose sum is σ = 21871539616. Its totient is φ = 21166005960.

The previous prime is 21518772749. The next prime is 21518772823. The reversal of 21518772787 is 78727781512.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21518772787 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21518772727) 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, 176383323 + ... + 176383444.

It is an arithmetic number, because the mean of its divisors is an integer number (5467884904).

Almost surely, 2^21518772787 is an apocalyptic number.

21518772787 is a deficient number, since it is larger than the sum of its proper divisors (352766829).

21518772787 is an equidigital number, since it uses as much as digits as its factorization.

21518772787 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 352766828.

The product of its digits is 3073280, while the sum is 55.

The spelling of 21518772787 in words is "twenty-one billion, five hundred eighteen million, seven hundred seventy-two thousand, seven hundred eighty-seven".