1991561 has 16 divisors (see below), whose sum is σ = 2466240. Its totient is φ = 1581120.

The previous prime is 1991551. The next prime is 1991573. The reversal of 1991561 is 1651991.

It is a happy number.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1991561 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991551) 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 15 ways as a sum of consecutive naturals, for example, 2351 + ... + 3083.

It is an arithmetic number, because the mean of its divisors is an integer number (154140).

2^1991561 is an apocalyptic number.

1991561 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

1991561 is a deficient number, since it is larger than the sum of its proper divisors (474679).

1991561 is a wasteful number, since it uses less digits than its factorization.

1991561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 776.

The product of its digits is 2430, while the sum is 32.

The square root of 1991561 is about 1411.2267712880. The cubic root of 1991561 is about 125.8146472650.

Adding to 1991561 its product of digits (2430), we get a palindrome (1993991).

It can be divided in two parts, 1991 and 561, that added together give a palindrome (2552).

The spelling of 1991561 in words is "one million, nine hundred ninety-one thousand, five hundred sixty-one".