1661991661 has 8 divisors (see below), whose sum is σ = 1891911744. Its totient is φ = 1445209920.

The previous prime is 1661991637. The next prime is 1661991677.

It is a happy number.

1661991661 is nontrivially palindromic in base 10.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 1661991661 - 2⁷ = 1661991533 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1661991601) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3284316 + ... + 3284821.

It is an arithmetic number, because the mean of its divisors is an integer number (236488968).

Almost surely, 2^1661991661 is an apocalyptic number.

1661991661 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

1661991661 is a deficient number, since it is larger than the sum of its proper divisors (229920083).

1661991661 is a wasteful number, since it uses less digits than its factorization.

1661991661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6569171.

The product of its digits is 104976, while the sum is 46.

The square root of 1661991661 is about 40767.5319463909. The cubic root of 1661991661 is about 1184.5214969440.

The spelling of 1661991661 in words is "one billion, six hundred sixty-one million, nine hundred ninety-one thousand, six hundred sixty-one".