1665561509921 has 2 divisors, whose sum is σ = 1665561509922. Its totient is φ = 1665561509920.

The previous prime is 1665561509897. The next prime is 1665561510017. The reversal of 1665561509921 is 1299051655661.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 1502132128225 + 163429381696 = 1225615^2 + 404264^2 .

It is a cyclic number.

It is not a de Polignac number, because 1665561509921 - 2²⁶ = 1665494401057 is a prime.

It is not a weakly prime, because it can be changed into another prime (1665561589921) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 832780754960 + 832780754961.

It is an arithmetic number, because the mean of its divisors is an integer number (832780754961).

Almost surely, 2^{1665561509921} is an apocalyptic number.

It is an amenable number.

1665561509921 is a deficient number, since it is larger than the sum of its proper divisors (1).

1665561509921 is an equidigital number, since it uses as much as digits as its factorization.

1665561509921 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 4374000, while the sum is 56.

The spelling of 1665561509921 in words is "one trillion, six hundred sixty-five billion, five hundred sixty-one million, five hundred nine thousand, nine hundred twenty-one".