21555660721 has 8 divisors (see below), whose sum is σ = 25931622080. Its totient is φ = 17503844688.

The previous prime is 21555660701. The next prime is 21555660727. The reversal of 21555660721 is 12706655512.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21555660721 - 2¹³ = 21555652529 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21555660727) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 81036186 + ... + 81036451.

It is an arithmetic number, because the mean of its divisors is an integer number (3241452760).

Almost surely, 2^21555660721 is an apocalyptic number.

It is an amenable number.

21555660721 is a deficient number, since it is larger than the sum of its proper divisors (4375961359).

21555660721 is a wasteful number, since it uses less digits than its factorization.

21555660721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 162072663.

The product of its (nonzero) digits is 126000, while the sum is 40.

It can be divided in two parts, 21555 and 660721, that added together give a square (682276 = 826²).

The spelling of 21555660721 in words is "twenty-one billion, five hundred fifty-five million, six hundred sixty thousand, seven hundred twenty-one".