691561 has 4 divisors (see below), whose sum is σ = 744772. Its totient is φ = 638352.

The previous prime is 691553. The next prime is 691573. The reversal of 691561 is 165196.

691561 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is the 340-th star number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 281961 + 409600 = 531^2 + 640^2 .

It is not a de Polignac number, because 691561 - 2³ = 691553 is a prime.

It is a Duffinian number.

691561 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (691531) 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, 26586 + ... + 26611.

It is an arithmetic number, because the mean of its divisors is an integer number (186193).

2⁶⁹¹⁵⁶¹ is an apocalyptic number.

It is an amenable number.

691561 is a deficient number, since it is larger than the sum of its proper divisors (53211).

691561 is a wasteful number, since it uses less digits than its factorization.

691561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 53210.

The product of its digits is 1620, while the sum is 28.

The square root of 691561 is about 831.6014670502. The cubic root of 691561 is about 88.4321460589.

The spelling of 691561 in words is "six hundred ninety-one thousand, five hundred sixty-one".