1661985121 has 4 divisors (see below), whose sum is σ = 1663258032. Its totient is φ = 1660712212.

The previous prime is 1661985047. The next prime is 1661985151. The reversal of 1661985121 is 1215891661.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 1215891661 = 18443 ⋅65927.

It is a cyclic number.

It is not a de Polignac number, because 1661985121 - 2⁷ = 1661984993 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1661985151) 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, 634495 + ... + 637108.

It is an arithmetic number, because the mean of its divisors is an integer number (415814508).

Almost surely, 2^1661985121 is an apocalyptic number.

It is an amenable number.

1661985121 is a deficient number, since it is larger than the sum of its proper divisors (1272911).

1661985121 is a wasteful number, since it uses less digits than its factorization.

1661985121 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1272910.

The product of its digits is 25920, while the sum is 40.

The square root of 1661985121 is about 40767.4517354225. The cubic root of 1661985121 is about 1184.5199432296.

The spelling of 1661985121 in words is "one billion, six hundred sixty-one million, nine hundred eighty-five thousand, one hundred twenty-one".