21557681786 has 4 divisors (see below), whose sum is σ = 32336522682. Its totient is φ = 10778840892.

The previous prime is 21557681771. The next prime is 21557681801. The reversal of 21557681786 is 68718675512.

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

It is an interprime number because it is at equal distance from previous prime (21557681771) and next prime (21557681801).

It can be written as a sum of positive squares in only one way, i.e., 21090300625 + 467381161 = 145225^2 + 21619^2 .

It is a super-2 number, since 2×21557681786² (a number of 21 digits) contains 22 as substring.

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5389420445 + ... + 5389420448.

Almost surely, 2^21557681786 is an apocalyptic number.

21557681786 is a deficient number, since it is larger than the sum of its proper divisors (10778840896).

21557681786 is a wasteful number, since it uses less digits than its factorization.

21557681786 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10778840895.

The product of its digits is 5644800, while the sum is 56.

The spelling of 21557681786 in words is "twenty-one billion, five hundred fifty-seven million, six hundred eighty-one thousand, seven hundred eighty-six".