21205661606501 has 4 divisors (see below), whose sum is σ = 21936891317100. Its totient is φ = 20474431895904.

The previous prime is 21205661606491. The next prime is 21205661606513. The reversal of 21205661606501 is 10560616650212.

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

It can be written as a sum of positive squares in 2 ways, for example, as 8890833062500 + 12314828544001 = 2981750^2 + 3509249^2 .

It is a cyclic number.

It is not a de Polignac number, because 21205661606501 - 2¹⁰ = 21205661605477 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21205661603501) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 365614855256 + ... + 365614855313.

It is an arithmetic number, because the mean of its divisors is an integer number (5484222829275).

Almost surely, 2^{21205661606501} is an apocalyptic number.

It is an amenable number.

21205661606501 is a deficient number, since it is larger than the sum of its proper divisors (731229710599).

21205661606501 is an equidigital number, since it uses as much as digits as its factorization.

21205661606501 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 731229710598.

The product of its (nonzero) digits is 129600, while the sum is 41.

The spelling of 21205661606501 in words is "twenty-one trillion, two hundred five billion, six hundred sixty-one million, six hundred six thousand, five hundred one".