21681418381561 has 3 divisors (see below), whose sum is σ = 21681423037893. Its totient is φ = 21681413725230.

The previous prime is 21681418381523. The next prime is 21681418381573. The reversal of 21681418381561 is 16518381418612.

The square root of 21681418381561 is 4656331.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is not a de Polignac number, because 21681418381561 - 2¹¹ = 21681418379513 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21681418331561) by changing a digit.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 2328166 + ... + 6984496.

It is an arithmetic number, because the mean of its divisors is an integer number (7227141012631).

Almost surely, 2^{21681418381561} is an apocalyptic number.

21681418381561 is the 4656331-st square number.

21681418381561 is the 2328166-th centered octagonal number.

It is an amenable number.

21681418381561 is a deficient number, since it is larger than the sum of its proper divisors (4656332).

21681418381561 is an frugal number, since it uses more digits than its factorization.

21681418381561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9312662 (or 4656331 counting only the distinct ones).

The product of its digits is 2211840, while the sum is 55.

The spelling of 21681418381561 in words is "twenty-one trillion, six hundred eighty-one billion, four hundred eighteen million, three hundred eighty-one thousand, five hundred sixty-one".