21015666591 has 4 divisors (see below), whose sum is σ = 28020888792. Its totient is φ = 14010444392.

The previous prime is 21015666589. The next prime is 21015666613. The reversal of 21015666591 is 19566651012.

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 21015666591 - 2¹ = 21015666589 is a prime.

It is a super-2 number, since 2×21015666591² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21015666521) 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, 3502611096 + ... + 3502611101.

It is an arithmetic number, because the mean of its divisors is an integer number (7005222198).

Almost surely, 2^21015666591 is an apocalyptic number.

21015666591 is a deficient number, since it is larger than the sum of its proper divisors (7005222201).

21015666591 is an equidigital number, since it uses as much as digits as its factorization.

21015666591 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7005222200.

The product of its (nonzero) digits is 97200, while the sum is 42.

The spelling of 21015666591 in words is "twenty-one billion, fifteen million, six hundred sixty-six thousand, five hundred ninety-one".