668401991 has 2 divisors, whose sum is σ = 668401992. Its totient is φ = 668401990.

The previous prime is 668401963. The next prime is 668401999. The reversal of 668401991 is 199104866.

668401991 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 668401991 - 2¹⁸ = 668139847 is a prime.

It is a super-2 number, since 2×668401991² = 893522443145528162, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (668401999) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 334200995 + 334200996.

It is an arithmetic number, because the mean of its divisors is an integer number (334200996).

Almost surely, 2^668401991 is an apocalyptic number.

668401991 is a deficient number, since it is larger than the sum of its proper divisors (1).

668401991 is an equidigital number, since it uses as much as digits as its factorization.

668401991 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 93312, while the sum is 44.

The square root of 668401991 is about 25853.4715463901. The cubic root of 668401991 is about 874.3377807402.

The spelling of 668401991 in words is "six hundred sixty-eight million, four hundred one thousand, nine hundred ninety-one".