1666439705521 has 4 divisors (see below), whose sum is σ = 1666531020532. Its totient is φ = 1666348390512.

The previous prime is 1666439705519. The next prime is 1666439705531. The reversal of 1666439705521 is 1255079346661.

It is a happy number.

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 661032415521 + 1005407290000 = 813039^2 + 1002700^2 .

It is a cyclic number.

It is not a de Polignac number, because 1666439705521 - 2¹ = 1666439705519 is a prime.

It is a super-3 number, since 3×1666439705521³ (a number of 38 digits) contains 333 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1666439705531) 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, 45630126 + ... + 45666631.

It is an arithmetic number, because the mean of its divisors is an integer number (416632755133).

Almost surely, 2^{1666439705521} is an apocalyptic number.

It is an amenable number.

1666439705521 is a deficient number, since it is larger than the sum of its proper divisors (91315011).

1666439705521 is an equidigital number, since it uses as much as digits as its factorization.

1666439705521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 91315010.

The product of its (nonzero) digits is 8164800, while the sum is 55.

It can be divided in two parts, 166643 and 9705521, that added together give a square (9872164 = 3142²).

The spelling of 1666439705521 in words is "one trillion, six hundred sixty-six billion, four hundred thirty-nine million, seven hundred five thousand, five hundred twenty-one".