1665561509920 has 192 divisors, whose sum is σ = 4906695128832. Its totient is φ = 519045120000.

The previous prime is 1665561509897. The next prime is 1665561509921. The reversal of 1665561509920 is 299051655661.

It is a Harshad number since it is a multiple of its sum of digits (55).

It is not an unprimeable number, because it can be changed into a prime (1665561509921) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 118277280 + ... + 118291360.

It is an arithmetic number, because the mean of its divisors is an integer number (25555703796).

Almost surely, 2^{1665561509920} is an apocalyptic number.

1665561509920 is a gapful number since it is divisible by the number (10) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 1665561509920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (2453347564416).

1665561509920 is an abundant number, since it is smaller than the sum of its proper divisors (3241133618912).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

1665561509920 is a wasteful number, since it uses less digits than its factorization.

1665561509920 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 23715 (or 23707 counting only the distinct ones).

The product of its (nonzero) digits is 4374000, while the sum is 55.

The spelling of 1665561509920 in words is "one trillion, six hundred sixty-five billion, five hundred sixty-one million, five hundred nine thousand, nine hundred twenty".