643561 has 4 divisors (see below), whose sum is σ = 646272. Its totient is φ = 640852.

The previous prime is 643553. The next prime is 643567. The reversal of 643561 is 165346.

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.

It is a cyclic number.

It is not a de Polignac number, because 643561 - 2³ = 643553 is a prime.

It is a Duffinian number.

643561 is a lucky number.

It is a nialpdrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (643567) 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 3 ways as a sum of consecutive naturals, for example, 961 + ... + 1486.

It is an arithmetic number, because the mean of its divisors is an integer number (161568).

2⁶⁴³⁵⁶¹ is an apocalyptic number.

It is an amenable number.

643561 is a deficient number, since it is larger than the sum of its proper divisors (2711).

643561 is a wasteful number, since it uses less digits than its factorization.

643561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2710.

The product of its digits is 2160, while the sum is 25.

The square root of 643561 is about 802.2225377038. Note that the first 3 decimals coincide. The cubic root of 643561 is about 86.3369241992.

The spelling of 643561 in words is "six hundred forty-three thousand, five hundred sixty-one".