667821561 has 32 divisors (see below), whose sum is σ = 1012248576. Its totient is φ = 387960000.

The previous prime is 667821551. The next prime is 667821563. The reversal of 667821561 is 165128766.

It is a de Polignac number, because none of the positive numbers 2^k-667821561 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (667821563) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 106411 + ... + 112511.

It is an arithmetic number, because the mean of its divisors is an integer number (31632768).

Almost surely, 2^667821561 is an apocalyptic number.

It is an amenable number.

667821561 is a deficient number, since it is larger than the sum of its proper divisors (344427015).

667821561 is a wasteful number, since it uses less digits than its factorization.

667821561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6253.

The product of its digits is 120960, while the sum is 42.

The square root of 667821561 is about 25842.2437299860. The cubic root of 667821561 is about 874.0846202893.

It can be divided in two parts, 667 and 821561, that added together give a palindrome (822228).

The spelling of 667821561 in words is "six hundred sixty-seven million, eight hundred twenty-one thousand, five hundred sixty-one".