666401 has 4 divisors (see below), whose sum is σ = 669252. Its totient is φ = 663552.

The previous prime is 666353. The next prime is 666403. The reversal of 666401 is 104666.

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 564001 + 102400 = 751^2 + 320^2 .

It is a cyclic number.

It is not a de Polignac number, because 666401 - 2¹⁴ = 650017 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (666403) 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, 1040 + ... + 1553.

It is an arithmetic number, because the mean of its divisors is an integer number (167313).

2⁶⁶⁶⁴⁰¹ is an apocalyptic number.

It is an amenable number.

666401 is a deficient number, since it is larger than the sum of its proper divisors (2851).

666401 is a wasteful number, since it uses less digits than its factorization.

666401 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2850.

The product of its (nonzero) digits is 864, while the sum is 23.

The square root of 666401 is about 816.3338777731. Note that the first 3 decimals coincide. The cubic root of 666401 is about 87.3464408714.

The spelling of 666401 in words is "six hundred sixty-six thousand, four hundred one".