643821 has 4 divisors (see below), whose sum is σ = 858432. Its totient is φ = 429212.

The previous prime is 643781. The next prime is 643847. The reversal of 643821 is 128346.

It is a happy number.

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 643821 - 2⁷ = 643693 is a prime.

It is a D-number.

643821 is a lucky number.

It is a junction number, because it is equal to n+sod(n) for n = 643791 and 643800.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (643021) 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, 107301 + ... + 107306.

It is an arithmetic number, because the mean of its divisors is an integer number (214608).

2⁶⁴³⁸²¹ is an apocalyptic number.

It is an amenable number.

643821 is a deficient number, since it is larger than the sum of its proper divisors (214611).

643821 is a wasteful number, since it uses less digits than its factorization.

643821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 214610.

The product of its digits is 1152, while the sum is 24.

The square root of 643821 is about 802.3845711378. The cubic root of 643821 is about 86.3485494002.

The spelling of 643821 in words is "six hundred forty-three thousand, eight hundred twenty-one".