• 640021² = 409626880441 is an anagram of the next square 640022² = 409628160484.

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640021 has 4 divisors (see below), whose sum is σ = 667872. Its totient is φ = 612172.

The previous prime is 640019. The next prime is 640027. The reversal of 640021 is 120046.

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 640021 - 2¹ = 640019 is a prime.

It is a Duffinian number.

640021 is a lucky number.

It is a nialpdrome in base 15.

It is a junction number, because it is equal to n+sod(n) for n = 639983 and 640010.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640027) 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, 13891 + ... + 13936.

It is an arithmetic number, because the mean of its divisors is an integer number (166968).

2⁶⁴⁰⁰²¹ is an apocalyptic number.

It is an amenable number.

640021 is a deficient number, since it is larger than the sum of its proper divisors (27851).

640021 is a wasteful number, since it uses less digits than its factorization.

640021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 27850.

The product of its (nonzero) digits is 48, while the sum is 13.

The square root of 640021 is about 800.0131248923. The cubic root of 640021 is about 86.1783301561.

Adding to 640021 its reverse (120046), we get a palindrome (760067).

The spelling of 640021 in words is "six hundred forty thousand, twenty-one".