1661001991 has 8 divisors (see below), whose sum is σ = 1812366864. Its totient is φ = 1509697920.

The previous prime is 1661001967. The next prime is 1661002009. The reversal of 1661001991 is 1991001661.

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

It is a cyclic number.

It is not a de Polignac number, because 1661001991 - 2²⁹ = 1124131079 is a prime.

It is a super-2 number, since 2×1661001991² = 5517855228211928162, which contains 22 as substring.

1661001991 is a strobogrammatic number because it is the same when read upside-down.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1661002991) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 56761 + ... + 80893.

It is an arithmetic number, because the mean of its divisors is an integer number (226545858).

Almost surely, 2^1661001991 is an apocalyptic number.

1661001991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

1661001991 is a deficient number, since it is larger than the sum of its proper divisors (151364873).

1661001991 is a wasteful number, since it uses less digits than its factorization.

1661001991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 30401.

The product of its (nonzero) digits is 2916, while the sum is 34.

The square root of 1661001991 is about 40755.3921708527. The cubic root of 1661001991 is about 1184.2863336793.

The spelling of 1661001991 in words is "one billion, six hundred sixty-one million, one thousand, nine hundred ninety-one".