21661465736561 has 16 divisors (see below), whose sum is σ = 24788799360000. Its totient is φ = 18542348962368.

The previous prime is 21661465736557. The next prime is 21661465736591. The reversal of 21661465736561 is 16563756416612.

It is not a de Polignac number, because 21661465736561 - 2² = 21661465736557 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21661465736561.

It is not an unprimeable number, because it can be changed into a prime (21661465736531) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 36123815 + ... + 36718563.

It is an arithmetic number, because the mean of its divisors is an integer number (1549299960000).

Almost surely, 2^{21661465736561} is an apocalyptic number.

It is an amenable number.

21661465736561 is a deficient number, since it is larger than the sum of its proper divisors (3127333623439).

21661465736561 is an equidigital number, since it uses as much as digits as its factorization.

21661465736561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 601648.

The product of its digits is 32659200, while the sum is 59.

The spelling of 21661465736561 in words is "twenty-one trillion, six hundred sixty-one billion, four hundred sixty-five million, seven hundred thirty-six thousand, five hundred sixty-one".